Sprinting isn't perfect, but it sure rocks! It builds strength, makes the metabolism skyrocket, burns fat, conditions the body for most sports and much more. Hill sprints are great, because they have a much lower injury risk over flat ground sprinting and are quite a bit harder, so I recommend hill sprints over flat ground unless to are training for a sport that requires a lot of sprinting, then you may want to incorporate both flat ground and hill both.
Make sure you warm up and do some mobility work before you do sprints. Especially loosen up the hip flexors, those tend to be tight and weak... and are a key factor to sprinting!
I recommend sprinting 3-4 days a week if you're goal is to get lean and fast. 2-3 days a week if you want to gain weight. 3-5 if you don't do much else for exercise. You really don't need much more if you don't have big fitness goals like myself.
Now, go make sprinting a habit!
Just a word of warning: if you're training earlier in the morning or in the winter, get really warm and mobile inside first. You may need a sweater and running pants. Light pants are better. Having heavy pants can mess with your form or over stress the hip flexor.
Have at It! I plan on doing some this week! I restarted my fitness quest last week!
