100mi as last long ride for IM

Four weeks before your IM?  There's no problem at all.  I did similar things last year with good results.

4 weeks out is a great time for a ride of that distance.  for me, long rides like that are tollerated much better in social situations i.e. organized century rides, races, groups or buddies, etc.  I'd say go for it!

well it is not really a "normal" 100mi bike race
see the profile here
It is part of the today' Giro d'Italia course plus part of the tomorrow' course
there is the famous Mortirolo and Gavia passes
I hope to be able to climb Mortirolo with my compact 34-28 gear ratio. I am quite scared...

that looks like a great ride.  plenty of elevation to challenge you.  You'll likely be on the bike for more than 6 or 7 hours.  If you're prepared for that ride, you're in shape to do any ironman bike course.

I'll always do multiple 100-120+ mile rides in prep for an IM, but none are as hilly as the one you are going to do.  Again, if you train at a level you can do that ride, I think it'll be that much more volume to your benefit.  If you're not and it breaks you and demoralizes you, then it wouldn't be that helpful for overcoming mental obstacles.

It will be great physical training, and will give you a huge mental boost.

4 weeks out is plenty of time to recover.

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