The 100-400 is push pull for the zoom and when it's all the way out it I suppose it could get heavy for some but I've hiked with it and never felt uncomfortable using it. I also think that it is something that you get accustomed to handling and becomes 2nd nature after a while.
For shorter distances I use the EF 28-70mm 1:2.8 (it does not have IS (image stabilization) as it was not available when I bought it)
Selecting the best lens combination is a little different for each person depending on what you are shooting. If you get the 100-400 you'll have your wildlife lens covered. Some people love to use prime lenses as they are much less money. I have a EF 20mm 1:2.8 and really like it for taking those wide angle shots and it's very small in size and was not anywhere near the price of the zoom lenses. More than likely you'll start out with the first 2 lenses and later you'll get the bug for another one or two
Investing into these quality lenses will be something you'll have when you buy the next camera body a few years from now. I started with D60's, then bought a 10D and now have the EOS1 Mark II and the lenses have been completely interchangeable between each camera body.
