Besides the boots, it would also be an idea to wear some mail. Its sounds a bit overkill, but you can easily shear through the bone with even a light glance with a decent axe. You can make a simple wrap around piece of mail, with some decent wire netting. Just check it with an older axe to get a feel for its cut resistance.
One of the biggest problems with glances is going too fast, or working with an axe that is too thick and you are cutting on too low of an angle. In the beginning you should be very slow, moving hit by hit. Make sure your footing is very secure and the wood will take the stress of being hit. I would also stay well clear of trees that are not vertical for a long time.
As Jimbo noted, trees that are leaning are under great stress and much care has to be taken when dealing with them. You can check this yourself by cutting some small saplings with a small knife while bending them wither your other hand. You will note the rather extreme levels of penetration as well as how violent the cuts can be induced to run.
Felling problematic trees for exercise to build strength and endurance, I would put on par with running from the Bulls in spain to increase your 100 m sprint. It will work, but it is not trivial to do without danger, and even those experienced and skilled get seriously injured and even killed every year. There are far safer ways to get healthier. Of course vertical clean wood is much easier to fell and much safer, but it is not trivial for a novice to always pick out the safe trees.
I would be interested in some threads along those lines Jimbo, there is little discussion about axe use on the net now. The next time you are cutting once of the leaners it would be very nice if you took some shots and outlined the process.
-Cliff