Lots of great advice already given. I would only add, based on your last comment about blisters, that yes, your feet do change. Old boots aren't going to work for you anymore. Get something new that fits your old feet, something perhaps lighter, with a good insole. Got to keep those feet happy! They are your foundation and if they aren't comfortable, you're not going to be.
Don't try to push it too hard. As balzaccom mentioned, Our bodies don't recover as quickly and pushing them harder usually ends up working against us rather than helping.
Good luck!
Bill