thoughts on breaking up a long workout

Yes, You will get as much benefit from breaking it up as doing it on one block.  

I have seen some plan for time crunched people who want to do a 140.6 mi Triathlon that put a 3 hour bike ride on Friday and a 3 hour bike ride on Saturday rather than putting the full 6 hours on Saturday to make the training blocks easier to fit into busy schedules.  The plan claims that doing these endurance workouts back-to-back days without a recovery day in between is about the same benefit and doing a single block.

Also, there has been research done that shows that for Ultra Marathon run training that athletes actually get a greater training benefit from increasing frequency and decreasing duration.  They showed that greater gain come from doing stuff like 6 miles in the AM and 6 miles in the PM than by doing a single 12 mile run. 

So, you are going to be just fine on race day with splitting up your ride.