Starting is always the hardest part. It's OK to start really really small. I'm talking small, like try just standing up at first, then take 10 steps toward that bike or treadmill, then step on and give it 3 minutes, just 3. And after that you might finally feel like you can keep going so give it 3 more then 3 more then go pick up some weights and do only 3 reps. By now you're probably feeling a little better and if you can keep moving, pretty soon an hour will have gone by and you'll feel amazing. And someday you might be like this guy who rides a monster unicycle up a mountain and passes me during a bike race.