It also depends on where you start I think. If you are coming from a place of very low muscle mass for your frame it will take a while to get started (by trying to get the body used to working out and actually USING the strength in those muscles) then you can gain muscle fairly rapidly until you get to a normal level and then it gets hard again.
MTA: because it is rare I dont have more to say
"When I got too tired to run anymore I just pretended I wasnt tired and kept running anyway" - my daughter Age 7