Despite not being physically in shape myself (which I am working on), I live a very active lifestyle. I work in a kitchen so that involves a lot of walking, lifting and moving things, and as well I go to the gym several times a week, and I've found that since I've been working out and losing a bit of weight, my erections last a lot longer and they feel more hard. I also pump with the Bathmate X30, when I'm able to, and that also really helps. I think the combination of both exercise, a decent diet and pumping really help, for me at least. Everyone's body is different.
I've also found that after ejaculating I can keep going. A few months ago there would have been no chance of that happening, but I stay hard for a while afterwards and I can continue, whether it's sex or masterbating.