Make them run on a treadmill. Hold a knife, a tazer, or burning hot metal behind them. If they get tired and slow down, they get stabbed, shocked, or burnt.
You can do the same with other exercises as well. Make the machines speed up after a bit so the victim doesn't get used to it. You should probably alternate between knives and tazers and such so the victim cannot develop endurance.
Oh! And then there's adrenaline. Give them a shot of that when they are tired and they'll surely perk up.
Make sure you over-exercise every muscle they have for maximum pain.
And we cannot forget exercise music. Turn on some Justin Beiber and Rebecca Black full blast. While the victim suffers from the music as well, you will be safe with your earplugs.
For the treadmil, you can but something they have a phobia of behind them. In my case, it would be a dog. My torturer would hold the dog at my heels with a leash, meaning that the dog will get me if I'm to slow.
Then there's threats. Effective threats may include:
1) Run faster or else.
2) Faster, or you will be severely [insert torture here] later!
3) Don't make me get the [insert animal or bug they have a phobia of]!
4) You just added x minutes to your exercise time for being so slow.
5) I'll kill [insert person dear to victim] if you don't hurry up.
Random treadmill ideas:
1) Turn it on a speed that's easy at first, but difficult after a while. Then tie their arms to the treadmil and leave them all night.
2) Refuse to give them water until they are about to literally pass out.
3) When you do give them water, make them drink it by the jugfull and do not allow them to use the bathroom. They must then try to hold it as they run.
4) Just leave them there for as long as possible.
5) Cut them or something as they run, but make them keep running despite the pain.
6) Make sure you have a clock that makes noise every time the second hand moves. That'll make them nervous as heck, and they will be forced to constantly wonder how much longer the torture will last.
7) Make them run/walk on their knees, or make them crawl.
8) Sexually harrass them a little as they run.
9) Make them have to run fast enough to grab things nearby.
10) When it's all over and they aren't dead, make them thank you.