I too had extreme lightheadedness when I first started riding again. I attribute it to several things:

First and foremost, lack of aerobic fitness means that blood in the legs is slow to circulate back to the heart-- indeed, what bodybuilders call 'the pump' means that an awful lot of blood volume is temporarily in the legs after a good ride. Staying fully hydrated, spinning for a respectable distance towards the end of your ride (the 'cool down'), and stretching afterwards will certainly help with recuperation.

Next, a drop in one's resting heart rate as one becomes more fit can certainly be a factor...

and last, if one trains a lot in the anaerobic phase, or above one's lactate threshold, you're highly likely to bind a lot of blood volume in your legs as the muscles recuperate.

Dave