Hand numbness can be caused by a few things, vibration through the bike or too much forward tilt on the saddle for instance. None of us here can tell you for sure what it is over the internet. I'm sure most of us from time to time, good set up or not, have an issue with minor, short-lived numbness due to road vibration, though by your post it seems this is a common occurence on most rides. Is that right? If so, first place I'd look is at your saddle tilt. Too far down and you'll be trying to push yourself back on the seat constantly, possibly pinching nerves in the hand, leading to numbness. Too far up and you'll crush your genitals (numbness again). Everyone's different. Some will run a straight flat saddle, some with a bit of down tilt, others up. I personally have just a shade under 1/4 inch of nose up tilt because that's where I feel right. I recently had to grease the rails on the saddle, requiring me to take it off, and when I reinstalled, I had maybe 2mm more upwards tilt just from human error. I knew pretty quickly that I didn't like that, as I started feeling perineal pressure. Yeah, a couple millimeters can make that much of a difference.
Part two of your problem can also be due to a couple reasons. None of us can say if a 55 LeMond is right for you, as, again, everyone is different. My main bike is a 54 (measured center-top), but when measured against my 53 LeMond (measured center-center), it's appreciably longer, even though they're technically supposed to be same size (and LeMond's marketing hype of the "longer top tube than other brands" Yeah, okay). And according to Specialized's world, my bike should be fitted with a 120mm stem, but I felt off with it, and had them put a 90mm on. I guess what I'm driving at is that none of us know your set up, your body build or how you ride. You could be on a 150mm stem where a 100 would suffice for all we know and have your seat too low, both of which could be causing back issues. I'll tell you this- there are times if I get a little crazy and push too hard I can get muscular pain and stiffness in my back after any amount of time. This is almost definitely due to my lack of core strength. I'm the first to admit I need to do some weight work and probably some situps to strengthen my back and abs. Stretched out along your bike for long periods of time the only thing holding you up and still is your lower back and your abs. And you'd be surprised at how much work your back actually does for you on the bike. You give it too much juice too early and you could very well be causing the back pain. Make sure you're stretching as well. Lack of flexibility can cause big problems, especially in the lower back and legs. I'd be willing to bet with some core strengthening and some increased flexibilty the pain will stop.
