If you were going longer I'd suspect dehydration, but no way you can get dehydrated in eight miles unless the temp and humidity are both in the 90s.
It's vaguely possible you're OVER-hydrating, but more likely your riding position is too awkward -- perhaps reaching forward too much (given your waist circumference & general condition), or perhaps you're seat's too low.
Find some sites on the web about bike fit and make sure your bike is reasonably sized and reasonably well-adjusted. Raise your handlebar a bit higher, if it's not already at the warning mark.
There are some medical conditions that can be "revealed" from more intense cycling, but not likely at the level you're doing.
One other possibility is cycling in too "high" a gear (on a multi-speed bike). This places unnecessary strain on the entire body, especially the knees, but also the leg and lower torso muscles. Generally (on the flat) one should adjust gearing so as to pedal at a rate above 60 RPM (ie, one complete revolution of the pedals per second), and experienced cyclists will tend to pedal at 70-90 RPM (for some reason getting slower as we age). Another way to gauge pedaling speed (that works pretty well across a range of effort levels) is that you should do at least one revolution of the pedal for each breath you take, and preferably two revolutions per breath. It's OK to slow RPMs somewhat on a hill, but don't go so slow that you're "grinding".
The two most common mistakes made by new cyclists are to have the seat too low and to pedal too slowly.