Sean,
Bummer to hear you crashed. Ouch.
Healing cracked bones takes a lot of energy. Good call dropping the 9 hour ride. Drop the 20 min test also. It is pointless to test when up front you know the result will be questionable. Duplicate Thurs ride on Fri instead of testing.
I would stick to the road or trainer for the next 2 weeks and give your poor body time to heal.
To carry fitness through to May 1st while “off plan” and in healing mode do short rides like the one scheduled for Thursday, 60 min ride with 4 x 1 min for the rest of this week.
Next week if your ribs allow you to breathe deeper you can stick to the ride effort and durations but keep them on the road. Ride your mtb on the road for optimal cushion from bumps and also to get ride time in on your race bike.
As it is your second time through this plan I wouldn't worry about missing out the longest rides – I'm sure you have great endurance. Focus on the shorter fitness interval type rides to keep your VO2max and threshold up. Endurance is sticky and takes months to lose but VO2max and threshold will start to slide after 10 days or so.
Lynda