I think I have read that exercising, as well as making you happy, relaxed,healthy etc, can help make a guy last longer? Is this true? Has anyone noticed any sizable differences?
Im guessing it cardio exercise that would do the trick?
I can't say for certain, but I guess if you are generally fit and healthy, you have better blood flow etc.
But to be honest, i'm not exactly the picture of health and I could last all night if I wanted to!
Maybe the men of LH could take it upon themselves to find out. Summer challenge and all of that. 
Speaking personally, I am pretty fit, and it doesn't make a difference to my endurance at that particular event :-)
The main factors seem to be the more obvious ones of how long it has been since the last time, and how relaxed I am at the time....
I can say without fear of contradiction that it does big time. I got back into running seriously about five years ago and my wife reckons I could go all night if I wanted to. Not that my wife would want me to.
I'm sure there was a blog post on lovehoney way back that found from surveys the only credible link between another factor and 'stamina' was cardio fitness. can't find the url though
To be clear, are you asking if it helps you go on as in the actual mechanics of it before getting tired, or as in delaying your orgasm?
You know when they say its not about the size but its how you use it. Now I cant say that everyone s like that but one girl i knew use to be married and her ex use to exercise(bodybuilding) and did alot of running but he was a one minute wonder. She put up with it for long time but ended up leaving him. So that is one example of keeping fit and healthy.
Now I have used chiina brush in the past and can swear by it as i went from a minute more than 30-40 minutes. I noticed one thing when i used it. There was a time when i use to do delivery work and it was tough and always kept me fit. When i used china brush at that time, i would be able to go on without losing erection. Now when i left that job and now whenever i use it, i cant go as hard as before. I can go hard and i mean really hard in small spells but after 5-10 minutes i would start losing erection cause of the stamina or tiredness. Than my partner would have to give me bj and get me hard and i will go for sometime and the same would happen again and if i go slow than i can go on for long. So stamina does help but not as much in that way where it would make you last long. Its more about the sensitivity and mind over matters.