People I have worked out with highly recommend hundredpushups.com, hundredsitups.com, and twohundredsquats.com. I am currently kettlebelling and doing Convict Conditioning. If you are in reasonable shape try any of Stu Smith's books or online articles at military.com. He was a SEAL officer who was assigned to the Naval Academy to prepare midshipmen for BUDS, and claims a 85% pass rate. His books are not easy, but the 12 week programs are tough, involving running, swimming, and calesthenics (no equipment required other than a track, a pool, and time). If you have some weights at home then P90X is popular and several co-workers have had success with it. All of these programs will provide daily schedules, and a daily commitment of 10- 75+ minutes a day, three to six days a week, for 2-4 months.
Advice from a retired SEAL and a retired Special Forces Sergeant Major as well as a Warrant Officer and SGM from the Unit formerly known as Delta Force all have the same advice for Rucking. All of them worked in assessment and selection for their former units.
1. Make sure your boots fit and are broken in.
2. Start slow. One day a week, no more than 3 miles a day for the first couple of months.
3. Don't run with a pack on. Try to go a little faster each week, 30 minute miles to start with are fine. Build up to 15 minute miles, and then add a mile a month.
4. Weights should be kept reasonable, 25-35 pounds to start with, and if you want to be psycho increase regularly.
5. If you are psycho, add another day every few months.
6. Change only one parameter (distance,frequency, speed, weight) per month.
These Former Acrion Guys all advised those rules to save your joints. Two of them are classified as disabled. They all have said that there are reasons that their former units did things a specific way, but that sports science is showing them better ways, and if they were mentoring folks with an interest in going into Special Operations, this is the advice the give to prepare.
pat