Regardless of the shape you're in (or think you're in), start at the first rung of the Introductory Ladder and work your way up. The pace at which you climb the ladder depends on your own body. If you're in reasonable shape to start with, you may progress through the introductory rungs rapidly, then slow down as you reach the higher, more difficult levels.
Just as it's important to start at rung 1, it's also important not to rush from rung to rung too quickly. Please spend at least five days on each rung as you advance; many people will prefer to spend at least a week per rung. When I was developing this plan, I used the following schedule, which I recommend.
Spend one week on each rung. On a given day of the week, for example Sunday, try the next higher rung. If it seems as easy as the current rung felt the week before, move up to that level. If you have difficulty completing any exercise of the new rung, or you can't complete it in 15 minutes, or you feel pain or exhaustion at that level, stay at your current level for another week.