A person pursuing holiness with all of his or her life force (and with the grace of God, acquired by a lively interior life of prayer and happy sacrifice) will be ever new and joyful even as their body falls apart.

I know a few people who have "taken" on this way, and who encourage it to others, largely through their example. 

One is an 80 year old priest of Opus Dei who smiles nearly all the time, works very long days, drives hundreds of miles a week, teaches, reads, hears hundreds of confessions a week, gives retreats, says Mass, visits the sick, prepares for talks and classes (heavy theology/philosphy). 

But he was a trained electrical engineer and he still stays current in that field too. 

And when you send him an email or voice mail you hear back usually within hours. 

His whole strength.   All to God, for others, cheerfully.

He seems to be about 21 years old in terms of vitality.   But he attributes it to his life of prayer.  He converts his whole day of work into prayer.