Just started methylphenidate 5 mg for fatigue take one at 8 and 1 feel good (awake) but by 6 tired & ready to go to bed. How do I make it last longer?

Ritalin (methylphenidate) Discuss with your prescribing doctor the extended-release forms of stimulants.
Change formulation. There are longer-acting (slow-release) formulations available. Examples are Focalin (dexmethylphenidate) xr and Concerta which are designed to last thru all the waking hours of the day. The good news is that you seem to have responded to a very long dose and it lasts longer than expected, thus you may benefit from "intermediate-acting" forms like Ritalin la or Metadate CD that tend to be cheaper.