Alan, I bought a Touche last summer to commute to London on - great bike - took to it like a dream. Rode it on a couple of club runs in the Autumn too, including a hilly alternative ride into Kent with Pete S - bit of a killer on steep hills - the only time you will feel you are using every muscle in your body to turn the crank at the slowest cadence you'll ever achieve - not a wise route for it really
I can't imagine you'll have a problem especially with your cycling performance
Though I think you might be being a bit brave taking it out with the 18s on you first ride. I don't think distance is a problem - it's getting a chance to rest your legs is the thing to watch out for.
To summarise my experience ...
Things I love about a fixie:
The smooth acceleration you can achieve
The more instant response (the chain is lot chunkier and doesn't have as much give or flexibility as a skinny 10 speed chain).
Not worrying about changing gear.
Simple mechanics - less noise too.
Seems to give a good work out - build up speed and decelerate more smoothly
Things I hate about a fixie:
Forgetting for an instant moment that you are on a fixie and stop pedalling.
Not being able to take a rest freewheeling down a hill - though the trick is to let you legs completely relax and let the pedals turn them for you.
Overdoing the cadence on a downhill run, realising you are not going to sustain it and then having to put on resistance to slow yourself down to a more comfortable rate. Using effort to slow yourself down doesn't seem right.
Stopping at lights or junctions and having to lift the back wheel to reset the leading pedal.
Squeazing through narrow gaps between curb and stationary cars and worrying about catching the pedal on the curb.
Whilst at pace, sticking your hand out to indicate you are turning - its not easy steering one handed looking over your shoulder and keeping the pedals turning. This is probably where you will have the strongest instinct to stop pedalling.
Not being able to balance on the wheel at lights - something you need to master to be a "cool dude" and to be a convincing "fakenger"
Having to remember to carry a spanner to take the wheel off to fix a puncture - did you buy one? - Pearsons did me for £20!! Couldn't be bothered to go shop around as I wanted to get out on the bike.