Well VFE, I was the same at 20-25 because I thought I was both immortal and invincible! Now, at over twice the age, I know different - having just experienced Rick Wakeman's problem. It wakes you up a bit.
The important thing about exercise is that you don't have to burn yourself out and fall over backwards, purple-faced. You can get a heart-rate monitor (about£50) discover your own beneficial heart rate range for exercise (preferably from a professional, but the monitor instruction leaflet will probably give you an idea) and work in that range. Mine is 86-123 and I generally hit about 105 - without collapsing afterwards.
Everyone is different of course, but a good guide is speech - if you can talk easily while exercising then you are probably not working hard enough. If you can't speak at all then you are doing more than you need. If you can talk, but have to pause a bit between breaths to speak, then you've got it about right.
I have been a bit of a couch potato in the past and absolutely loathed exercise, but now, to my amused surprise, I actually enjoy it!