David Coffron’s answer does better than this one, but just for the sake of listing out some more options (which are also compatible with the Quick trait he uses and that I forgot about).
2nd level: 1st-level monk/1st-level barbarian or druid
To run 26.2 miles in 2 hours, a 3.5e character needs to achieve an average movement speed of 115.3 feet for the entire 2 hours.
Any character with a movement speed of 30 feet can achieve the necessary speed just by running—without heavy armor or a heavy load, running quadruples your movement for a round, so you’d move 120 feet. The problem with running is that
A character with a Constitution score of 9 or higher can run for a minute without a problem. Generally, a character can run for a minute or two before having to rest for a minute.
A character can’t run for an extended period of time.
Attempts to run and rest in cycles effectively work out to a hustle.
The rules don’t really give any way around this; these just are what they are. In fact, there isn’t even anything saying that you can ignore these rules by simply being immune to fatigue or whatever—so even an undead or construct creature can’t run for more than a minute or two, nevermind the fact that they literally never get tired. That’s probably an oversight, but an optimization challenge has to be played by the rules, and this is what they are.
The next-best option, hustle, deals 1 point of nonlethal damage when you get into your second hour, at which point you are fatigued (and can no longer run at all). Ironically, that doesn’t really matter—run isn’t a useful option for us, and taking a single point of nonlethal damage isn’t going to stop anybody. Fatigue also applies −2 penalties to Strength and Dexterity, but we aren’t really using either anyway.
A character moving his or her speed twice in a single round [...] is hustling when he or she moves.
So as long as we have a movement speed of 60 feet, we can hustle at 120 feet per round and achieve our goal.
Feat-wise, we start with two as a human, and we could take Dash from Complete Warrior for +5 feet, and/or Fleet of Foot from Player’s Guide to Faerûn for +10 feet. Fleet of Foot is Forgotten Realms material, can only be taken at 1st level, and requires you to be an elf, half-elf, or human from one of a few specific regions in Faerûn. Luckily, the challenge specified human as the race, and no requirements on where they are from, so our human can be from the Shaar or Thindol and take Fleet of Foot.
Beyond those, we want fast movement, obviously, and a few classes get that at 1st level:
- Cleric, if they have the celerity domain, though it might be Su
- Ardent, if they have the freedom mantle, though it might be Su
- Druid, if they are a druidic avenger, though it might not stack with barbarian
Each of these offers +10 feet at 1st level, so if we took the first level of two of them, along with Fleet of Foot, that brings us up to the necessary 60 foot movement speed. Dash doesn’t really help with the primary—65 feet is overkill, while 55 feet won’t do it—so we could take a different feat instead, but since Fleet of Foot is the only feat I can find with +10 or more movement speed (at least, that isn’t supernatural), that doesn’t help us directly.
It does help us indirectly, though, because ardent, cleric, and druid each have potential problems.
Ardent and cleric get their movement speed from a psionic mantle and a deity’s domain, respectively, which makes them questionable: a quick check doesn’t find anything saying they’re supernatural, but I’m not certain there isn’t one hiding somewhere, and even if there isn’t, some might say that’s an oversight. For what it’s worth, there are several domains that explicitly say that the granted power is a supernatural ability—which may mean that celerity’s movement bonus, which does not say that, is not. There aren’t any mantles with similar wording, and the freedom mantle only gives the bonus while you have psionic focus—but nothing says that is supernatural, either, though it does require that you have at least 1 power point remaining. So maybe ardent or cleric is fine, but we have other options.
Druidic avenger doesn’t have those problems, since their fast movement is “identical to the barbarian ability of the same name,” and that is explicitly extraordinary (i.e. non-magical). The problem is whether or not that stacks if we take it alongside barbarian: Unearthed Arcana doesn’t cover that situation. And neither does the wording of the barbarian’s fast movement ability itself. So the question becomes whether or not the barbarian’s “fast movement” and the druidic avenger’s “fast movement [that is] identical to the barbarian ability of the same name” count as the “same source” for the bonus-stacking rules. No one has a hard-and-fast answer for that.
So the barbarian or druidic avenger have no problems separately, but may not work with each other. Ardent or cleric may not qualify at all. And we need two classes with +10 movement speed at 1st level.
Enter the monk, which gets fast movement, explicitly extraordinary, and even better, it’s explicitly an enhancement bonus to speed, rather than the barbarian’s untyped bonus—that means that we don’t have to worry about whether the two “fast movement” abilities are the same source. Problem: monk doesn’t get fast movement until 3rd level.
No problem: Carmendine Monk is a feat that makes us count as two levels higher for one of a selection of monk abilities, that includes fast movement. So with monk 1st we’d actually be monk 3rd for the purposes of fast movement, i.e. +10 feet, and that brings us to the 60 feet we need. We have to be a member of the Zealots of the Written Word, who are monks and followers of the Faerûn god Deneir. That should be fine for this challenge, as neither the monk’s fast movement nor the Carmendine Monk feat are supernatural in nature, and we’re already from Faerûn in order to take Fleet of Foot (neither the Shaar nor Thindol are exactly centers of Deneir worship, but the faith is still found in each).
In order to take Carmendine Monk, however, we have to be a monk, so since we get a feat at 1st and not at 2nd, we should actually start as a 1st-level monk, and then take our level of barbarian or druidic avenger. Barbarian is actually a bit of a problem because monks must be lawful and barbarians must not be lawful. Luckily for us, the chaos monk is found in Dragon vol. 335, and also gets the same fast movement ability, and since Deneir is Neutral Good there is no problem with us being a Chaotic Good 1st-level chaos monk/1st-level barbarian.
The other alternative, if barbarian is desired rather than druidic avenger, is the “swift and deadly hunter” variant druid from Unearthed Arcana instead of monk. Ironically, Unearthed Arcana does cover multiclassing between druidic avenger and “swift and deadly hunter,” and the fast movement officially has no problem stacking. The problem is whether or not such a druid can take Carmendine Monk, which requires being a member of the Zealots of the Written Word, who are “monks” devoted to Deneir, though nothing explicitly says you have to be a member of the monk class to join. Even if a swift and deadly hunter can join, whether or not counting two monk levels higher for fast movement is good enough for the swift and deadly hunter’s “fast movement (as monk)” feature is also unclear. This probably works, but then, in a sense, so did ardent or cleric, but since chaos monk and druidic avenger exist we don’t have to bother.
That means that in two hours we achieve 27.3 miles, or we complete the marathon in 1 hour, 55 minutes, 16.8 seconds.