Hi Kimba,
Sorry I don't know anything about formal training, so can't offer anythign there except that you can try running some local hills and get a feel of things.

As for race choices, I stand by my earlier statement that a mountain race need not deter you, and life it too short to keep worrying (not that I didn't worry before my first race but I was lucky in who I ended up taking advice from). Let me explain and also respond to Tom's earlier post:

Compare Inferno HM (21k + 2200m altitude gain) which equates to approx 43 flat kms, and therefore equivalent to a flat marathon. I would probably take just under 3h 30 mins for each of them, and the winners take approx 2h 15min for them. Which of them is "tougher" if both take roughly same effort?

In a mountain race, obviously weather can be a challenge; it gets very cold at 3000m alt. But look at the positive side: there is fresh mountain air; the cold prevents excessive sweating and is conducive for endurance running, and possibly offers great views and thrill; compare with a city mara where you are jostling about with massive crowds and sweating like a...well you know what .

I fully agree with Tom that everyone would do their bl**dy best in a race, but the point I was making was different. What I was referring to was, if you check out a tough mountain race like TAR, even the winner has to walk some bits; on steep sections it borders transition to rock-climbing. In such sections the speed is limited not by pure leg/lung power, but by one's attitude, skill, and survival instinct (compare with riding downhill in Alpine cycling races). Less obviously, such sections also gives respite to the body in ways that a flat run doesn't.

Back to your specific query: I think that broadly there are two types of races: fully supported (e.g. Inferno) and semi-autonomous (e.g. TAR, UTMB; read adventur's blogs to get a sense of what it means). In a semi-autonomous, you can be miles away from help and you are expected to possess atleast some basic survival skills. So I would recommend that you do atleast 1-2 fully supported mountain races in CH and then decide if you feel confident to take up longer races (which will invariably be semi-autonomous).

In short, I don't think going all the way to UK to run an ultra-distance flat run is the best or most efficient way to transition to a mountain race in CH.

Note: this assumes you intrinsically love mountains/hills; else, just ignore this post