I've only ever come across one effective method for getting very high fitness very quickly. It ain't easy and it ain't pretty, but the results are remarkable.
High intensity interval training. I have linked you the FAQ here, but a typical week's workout would be along these lines (assuming you have average-to-decent fitness already):
Monday - 10min warm up jogging/similar, 20mins full body HIIT, 40mins sprints/running HIIT. 10min warm down.
Tuesday - rest
Wednesday - 10min warm up jogging/similar, 20mins full body HIIT, 40mins sprints/running HIIT.
Thursday - rest
Friday - - 10min warm up jogging/similar, 20mins full body HIIT, 40mins sprints/running HIIT.
Saturday - long(ish) run, low pace. Typical example is a 10k at your half-marathon pace (so you are
not exhausted at the end, but definitely feel like a workout).
Sunday - rest.
Repeat.
The exercise in detail:
- the 10min warm up is just that. Nothing intensive or feeling difficult, getting the blood flowing and joints working.
- the 20mins full body HIIT should be structured along the lines of 20seconds work at 100% intensity, 10seconds complete rest. If I were you I would do this for 2 exercises in 4x5min blocks. Even with reasonable fitness, with proper full intensity it will be killed, so have the exercises different or one feeling "easier".
An example: burpees (high intensity!) and V-sits for 5mins. So you do 20sec burpees, rest 10sec, 20sec V-sits, rest 10sec. Repeat 4 more times. So that's 10 sets of each, only 3min20sec of work but it will feel like a lot more.
Take a few minutes rest if need be, then the next 5min block can be the two exercises clapping pressups and airsquats. Jumping lunges and jumping jacks. Squat thrusts and pressups. Lots of exercises you can do. 20min total bodyweight HIIT is very intensive, keep the tempo high, rest as much as you need to between the "blocks" but don't sell yourself short.
- The 30mins sprintwork/fartlek type is HIIT but not max effort attempts. Running is the easiest example. Run for 30sec-1min (depending upon fitness) at a pace you find hard running. Then slow to a jog for recovery for 30sec-1min (depending upon fitness). Repeat until the 30mins is up. Understandably your sprint speed/faster run will decrease steadily along this half hour but will boost your speed as much as keep your heart/respiration rate high.