Whenever I need to get someone in amazing shape really fast my go to weapon of choice is hill sprints. There's something also mythical in their simplicity and effectiveness, after all what better describes the attainment of a physical ideal than conquering a hill.
A well planned training cycle of hill sprints will obliterate fat and carve up your abs whilst add shapely muscle to your glutes and hamstrings super fast. That said, you'd be hard pressed not to see improvements in shoulders, arms and overall posture. Real sprinting is a total body movement, this isn't jogging. Hill sprints have the potential to generate substantial increases in natural growth hormones and greatly stimulate fast twitch muscle fibres that tend to decline with age. Bottomline; you can keep all your fancy anti-ageing biohacks, hill sprints trump them all.
For the vast majority of males wanting to get in Brad Pitt's iconic "Fight Club" condition I'd argue most could do it on hill sprints alone maybe with a smattering of "beach muscle" workouts. Women wanting that tight waist and athletic bubble but, crank up the weekly volume on hill sprints and your prayers will be answered.
Beyond the conditioning and aesthetic benefits hills offer something more; it's a mental challenge. Hill sprints have been a staple of NFL and MMA conditioning programs for years precisely because they expose the trainee to what hard athletic training really looks like. This has a carry over into the rest of your training that can produce exponential results. Physically and mentally it makes almost everything else seem relatively easy.
WHY HILLS AND NOT FLAT SPRINTS?
Whilst sprinting up a hill is obviously harder it's ironically much safer than flat surfaces. If you haven't sprinted since high school - and even if you have - the potential for hamstring tears and other injuries is quite high. This is one of those greater the risk, the greater the reward scenarios. Thankfully hills mitigate much of the potential risk by preventing you hitting top speed and putting you in a position where you are constantly attempting to accelerate. The added benefit is this position also promotes better sprinting mechanics and assists in developing the entire posterior chain.
A couple of non-negotiables for starting hill sprints are firstly, a hill (obviously) with a consistent incline you can sprint up for between 30-50 meters, secondly is the wherewithal to ease yourself in for the first few sessions. Always begin with a thorough warm up and mobility work. If you don't know how you could do worse than googling Joe Defranco's "agile 8" to get you started, it's simple, accessible and can be performed daily to reduce soreness as well.
Before performing hill sprints always do several shorter sprints of lesser intensity to focus on technique, get a sense of the incline and gauge what top speed will look like. For beginners I'd suggest a workout of 5-6 sprints of between 30-40m on a steep incline with pretty much complete recovery between. That means each sprint should look equally explosive, if speed drops off noticeably between sprints you either need to stop the session or take longer rest. Failing this means you'll get a training effect but not the one you're after. Focus on getting faster and more explosive on hills and the fat loss and muscle will take care of itself.
Two hill sessions per week will produce outstanding results in anyone if performed correctly. No need to overcomplicate things just slowly add volume until you hit between 10-12 sprints of equal intensity. If you're consistently getting faster and more explosive at that volume your body will have no option but to change, you just have to put in the work.