30 miles on an upright bike is something that I'd avoid personally - it really is the territory of the true hybrid. The problem isn't so much lack of speed but increased ass and back damage:
http://www.sheldonbrown.com/pain.html
Back
Back pain while cycling is usually caused by poor cycling posture. Good cycling posture is very different from good posture while sitting or standing. A posture that is comfortable for sitting still will not necessarily be comfortable while actually riding a bicycle. Correct cycling posture must facilitate the pedaling action, and also must enable the rider to cope with the jolts that result from road irregularities.
When riding a bicycle, the back should be arched, like a bridge, not drooping forward between the hips and the shoulders. If the back is properly arched, bumps will cause it to flex slightly in the direction of a bit more arch; this is harmless. If you ride swaybacked, bumps will cause the back to bow even farther in the forward direction, which can lead to severe lumbar pain.
Some back-pain sufferers modify their bicycles with extra-high handlebars so that they can sit bolt upright, with their spines straight. This is actually counterproductive in most cases, because a straight spine has no way to "give" when the bike hits bumps. Road irregularities will jam the vertebrae together, often aggravating existing back problems. The bolt-upright posture is comfortable if you're sitting stationary on the bike, but is not suitable for riding much faster than a brisk walk. Riders who for some reason require such a position should use some form of suspension...a sprung saddle at the very least.
I'd suggest fitting a suspension seat post these days. Re the ass-pain: avoid those super wide "comfort" saddles, get something like a WTB Speed V. Also uprights usually come with terrible tyres and brake pads. Change them for, oh, 38mm Marathons and Kool Stops. And BMX platform pedals give the best mix of comfort and efficiency.