Sgt L I trust that when you say a 3 hr "spin", you do actually mean that you've been on the road? In fact I find that a 3 hour road cycle is far and away easier than 3 hrs on the turbo. For one, your backside and your feet don't seem to take the same hammering. They are going to feel hammered after 7 hours on the bike though. Your race is in June isn't it? I presume that you are progressing up from the regular 60 mile to 80 and 100 mile bike rides pretty soon.
If you plan to shuffle the third part, and believe me, a strong walk is faster than a bad jog, again, there's a need to clock up some foot miles. Out there. In the rain, cold and hey! It's just turned spring! Sunshine.