Try this descending ladder workout to challenge tired legs
This time-based workout will help you learn how to speed up when the going gets tough

Pacing is a tricky thing, even for experienced runners. Ideally, you want to keep your pace even throughout most of your race and find an extra gear at the end to have a strong finish, but speeding up when your body is screaming at you to stop can sometimes feel impossible. This workout will help you practice that skill, so the next time you’re in the final kilometre of a race, you can bring the heat instead of fizzling at the finish line.
There are only four intervals in this workout, each one shorter than the last. The goal here is to speed up as the intervals get shorter, so that by the end you’re running pretty close to as fast as you can. For this reason, it’s important that you start out at a pace that allows you some room to speed up. If you’re new to interval training or at the beginning of a training cycle, try to stick to half-marathon pace for the first interval and speed up from there. If you’re in the middle of a training cycle and you’re very fit, consider starting out a little faster (at 10K pace).
The workout
Warmup: 15-20 minutes easy jog
Workout: 8 minutes hard, 1:30 easy; 6 minutes hard, 1:30 easy; 3 minutes hard, 1:30 easy, 1:30 minutes hard
Cooldown: 10-15 minute easy jog, followed by light stretching