Post the workout you're thinking about and I can give you some ideas.
I can't find a copy of it unfortunatly and have been doing a couple others that are pretty challenging.
The jist of the one I was talking about earlier is:
M I run long distance at 830 min/mile pace(3 miles increasing over time to 9) included with this is core, and back workouts
T swim constantly for a certian amount of time, legs push ups pull ups and sit ups
W running diff amount of sprints of varying distances, shoulder, core, and chest.
Th swim timed laps, push pull and sit ups, and legs again.
Fr run constantly for certain amounts of time, core, arms and something else I can't remember
Sat. swim a certain distance (500-1000 meters) push pull and sit ups.