Yes, you need to be really fit. It is physically demanding. You need to be able to run two miles, and do as many situps and pushups as you can in two minutes. Plus, you have to be able to carry a pack and a rifle. You can start now. Get a copy of the physical training standards for your age. If you are overweight by a few pounds, lose those few pounds. Eat well. Exercise at least every other day. Yes, a gym membership would help. The benefits are that this profession will help ensure that you live a healthy lifestyle! (P.S. You do NOT have to be Hercules!)
Yes, the Army can provide you with matchless experience and a good life. It’s very dependent, of course, on where you are asigned and where you are assigned. But I highly recommend it!