The Player's Handbook states that:
- a running long jump equals your Strength score in feet and,
- each foot of movement of such a jump is just that, 1 foot of your move, and
- your move is restrained by your speed.
I ran an encounter using Barlgura demons (MM p. 56). Their speed is 30 feet. They have an ability called "Running Leap" that states that their running long jump is 40 feet. Also I assume that the "running start" mentioned is at least 10 feet of movement, getting a minimum of 50 feet, 20 more than their speed allows.
An explanation of how this works would be appreciated, since I have a 20 Strength paladin now getting crazy ideas in that same campaign.