The target's speed increases by 10 feet until the spell ends.
Ten feet per round may not seem like much, but this spell has a great duration of one hour and is only first level. Even a fourth level caster, if you are willing to upcast with higher level slots, can use it for seven hours a day, or nine hours if they have arcane recovery or natural recovery. Applied over an eight hour march, and with your method of calculating speed, that nets you more than 18 extra miles of movement.
However, you should be aware that your DM may not be calculating your overland speed based on encounter speed. The PHB has rules for travel pace in miles per day irrespective of the speed of the traveler, but these assume "relatively simple terrain", travelers that will need to rest, and travelers that cannot move indefinitely at their dashing speed. Given you have a flying undead mount that is likely immune to exhaustion, your DM may apply the Special Travel Pace (DMG 242, 242) as noted by Darth Pseudonym. Following these, "In 1 hour, you can move a number of miles equal to your [non-dashing] speed divided by 10" and "For daily travel, multiply your hourly rate of travel by the number of hours traveled (typically 8 hours)" and "For a fast pace [which gives you a Penalty on Perception checks while traveling], increase the rate of travel by one-third."
Putting all of this together, your flying undead mount has speed of 50, thus moves at 5mph, and can cover 40 miles in a day, or c. 53 miles if moving at a fast pace. With eight applications of longstrider, this would increase to a speed of 60, 6pmh, 48 mpd, or 64 miles in a day moving fast, boosting your travel by nearly eleven miles (with each application of longstrider netting you a bit more than a mile as bonus travel).
As a bit of a frame challenge, suppose you wanted to move fast overland, and there was a spell that let you move faster - but not by making your undead mount fly faster. Rather, by making you move so fast that your undead mount is the slowest member of your party, and could actually be abandoned.
Phantom Steed creates a mount for you, and one that will let you move 13 miles an hour at a fast pace, which is more than double your undead mount, even with a longstrider on it. Phantom steed lasts an hour, but it is third level, so your 7th level wizard caster can get in four castings a day with an upcast (or up to 6 a day with an 8th level caster and arcane recovery).
However, unlike your undead mount, you will need a phantom steed for each party member - so even the four castings you can do at 7th level will last only an hour for a party of four - if they are hard cast. Phantom Steed is also a ritual spell, though - which means you can cast it without spending spell slots, and without even preparing it. But here's the tricky part - each steed you make will take 11 minutes, and if you want the party to travel together, you won't be able to leave until you have made steeds for everyone.
With a small party of three, it would take you 11 minutes from "start time" to make one steed, 22 minutes to make two, and 33 minutes to make the third. However, if you start immediately after completing the third, you will have only 38 minutes left to travel, since the steeds last only an hour and your first steed is at that point already 22 minutes old. Traveling at 13 miles an hour for those 38 minutes, the three of you together can cover 8.2 miles. In the same amount of time it took you to do that (71 minutes; 33 minutes to make the three steeds and then 38 minutes of traveling), your flying undead mount would have traveled 5.9 miles. Since casting the steeds as rituals cost you no spell slots, at the time the first one disappears you can begin the process over again, and thus cover much more ground than you could on your mount.
As you can see, there are diminishing returns to this. If you need to make four mounts for your party, then you will spend 44 minutes making them, and have only 27 minutes left to travel because the first mount is by then 33 minutes old. With four steeds, you would cover 5.85 miles before needing to recast, whereas your flying mount could still cover 5.9 miles in those 71 minutes. With greater party numbers your travel time and distance covered continues to diminish.
However, you can still hard cast to get a boost. Perhaps your party of four all gets steeds for the first hour with four casts in four minutes, and then ritual casts all the others. Or maybe you ritual cast three and then throw in a hard cast for the fourth person each time. The steeds are actually creatures, so you can cast longstrider on them as well for a further boost.
Long term goals
While not available at your current level, if you have a druid, at 11th they will be able to access wind walk, which is far superior to your undead mount and the phantom steeds.