In December I will start a marathon plan for a race on April 18th. I know it is bad timing but for May 23rd I have signed up for an olympic triathlon. How should I best transition to triathlon training after after the marathon race?
Assuming the marathon is more important than the Oly, the first step is to take advantage of the cross-train options embedded in the marathon plan. You can use those sessions to swim and bike, to keep yourself sharp in those two sports. Also consider “leveling up” one level above what you might normally do (Level 1 to Level 2, for example) so that you have more cross-train options and that you are still running sufficiently to support your marathon.
After the marathon, take a week off. Then load the Oly plan for the 4 weeks remaining, but no running for the first of that 4 weeks, just swim and bike (so two total weeks no running).
The Oly brick workouts over that final 4 weeks will be tough, you’ll want to perform the bike at low Zone 3, at least for the first session.
This plan will have you 100% prepared for the marathon, and 85-90% prepared for the Oly.
If the Marathon and Oly are equally important, this presents the need for a complete custom plan. The two sports are disparate enough that in order to fully peak for both within a 5 weeks period, it would require more than what we have available in our plan portfolio.