How much exercise is needed to maintain?

I am 44 to be 45 in a few months and have maintained my weight fro about a year.  I try to work out 5 to 6 days a week power walking and running/walk on the treadmill.  I lift weights for about 1/2 hour to 45 minutes 3 days a week.  But sometimes like now I get so exhausted I cant get up on the morning and shut off my alarm instead.  So at this age how much exercise is really needed to maintain.  I do 70 minutes on the treadmill a time and about 28 miles total.  I dont want to lose any ground I have made.

as long as you have found an activity level you can sustain, that is all that is required.  try stepping it back a little bit.  maybe 3 days of the walking, alternated with 3 days of weights, or just cut back the time on everything, until you find something you can live with.  i don't think you should exhaust yourself.    muscles do need rest and recovery time after a strenuous workout.  of course, you may have to cut back on calories a bit, and make sure you are getting enough protein.