I agree with Ayres, definitely try to rent, or borrow. Also, get some help at the store or from one of your experienced friends in adjusting the pack properly. This can make a huge difference.

By all means, get in some preliminary trips before hand - even an overnighter with negligible hiking will get you used to sleeping out and dealing with your gear. Other than hiking, distance running will help your wind and legs a lot.

Buying a pack of this type is a big investment - not just the cost, but also in getting a suitable pack with the right features which will feel comfortable and not beat you up. You are not likely to spend less than $200, and probably more, for a decent pack.

The last time I bought a pack like this, I tried on several models in stores, and rented two models for actual trips, taking about three months to make a selection. But the one I chose has served me well for over a decade.