You may end up being fed up with all my questions and few answers. So let me try to issue some recommendations given the info I've gathered so far.
OK.
1 - Running
Tom, have you ever thought about learning how to run? Because while you may think of these 6months as being a period of non-productivity, at least cycling wise, I see there a great opportunity to progressively get into the world of Running. Especially if you feel that this situation may occur again in the future, running comes with a lot of freedom. You can run everywhere in the world.
After this gentle conversion, you may envision doing some duathlon once in a while. Note that duathlon represents a great compromise between recreational cycling (which for some might not be challenging enough) and the nasty world of competitive cycling (which can at times be really really nasty e.g. Criteriums). The duathlon/triathlon community is basically very friendly, it's a familly and friends sort of mood.
As a running *primer*, I have an approach with which I had a lot of success in the past. Listen to this.
- 5min of light jog 3x / week. Doesn't sound to difficult does it?
- You increase by 5min per week (week#2 = 3x10min; #3 = 3x15min etc) Don't skip any step please
- You have to listen to the feed back you articulations are going to give you. It is particularly true with the front part of the lower leg (sorry for lack of anatomy, I am a frenchy. In french we call this portion Perioste). This is where pain usually shows up first with newbies. If it happens, then don't increase by 5min for this week.
- Once you reach 3x30min, keep increasing by 5min for 1 of the workout. This will become your long run. You can let it go up to 90min, even 120min if you are really gifted for running. Again, articulations are your coaches there. This is the major constraint to take into account.
At the end of this regiment, you can consider yourself as a runner. 'Course, you have to pay a visit to running shoes specialist (to find out if your leg profile is straight, slightly inside, or outside). Buy at least 2 pairs, and change every 3 months. Don't do anything other than running with 'em.
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2- Cycling (off bike)
As far as cycling is concern now. The type of performances you will have to achieve during your trip isn't that difficult to get even with prolonged period off bike. If there's anything, you are going to have to be more conservative than usual with the Pace.
You have to make sure that the pace you take allows for burning Fat as metabolic. And this will happen if you ride relatively slow. Already one week into your trip, your body will tell you if you can increase the pace or not. So first week (into the trip), be careful. Ride slow. Then if you feel your body agrees to it, then increase the pace.
In order to stay fit cycling wise without riding, I'd say you could include some daily training such as Squats with no weight (that'd be my #1 choice). If I was facing the same challenge I'd probably try to go up to sets of 50. Do them full (go really low).
My main off season preparation (for cycling) involves mostly climbing stairs (on a machine). So like others have suggested, if you could climb anything, that will help. I found out recently that carrying a back pack kinds of trigger the same sort of neck pain that I get from cycling. So if you want to avoid neck pain, and if you need to carry a back pack, then think of it as a training tool that may help avoiding neck detraining (if such a thing as neck detraining exists!).
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So to recap, if you feel like giving this learn-how-to-run strategy a try, and if you can climb anything 2 times a week, along with some squats done in the morning, then I believe you will be ready for this 600miler no problem.