It would be a good idea to the join the gym! i was training for 5 months before i signed up and i obviously got fitter! i've now been at the gym for 4 months and i feel 10x fitter than i was. I've also bulked up alot where its needed. Just keep running at a fast pace and try and hit the old free weights from time to time.. not to mention try and eat healthy.. Your nutrition is very important!