This feeling of lack of energy at this age can be because of the depression
. Depression can present in variuos ways especially in young age people. Other condition associated with depression like unemplyement and inproper life style can precipitate the condition further. Since you told that you walk 4-6 miles every day means that you are physically in good condition.
I would suggest you to try to start working some where and try to get involve in something so that you can get some employment and this will also improve of general condtition. In the mean time try to have positive attitude towards every thing and start working hard to get something for you.
I know it is very difficult to get something of your interest and something ehich you like to work, but start something, because if you are free and have nothing to do than this will worsen your condition and apart form this try to keep a routine life.
If you can get a chance try to have consulation with some psychiatrist
and disuss all this with him.
Thanks and take care.