Cycling in Cornwall is great if you like a challenge
You would be better off starting in the summer months IMO, then you will get fit before you have to deal with the darkness, muddy roads and hard rain and wind all together. Apart from the previous too winters, snow has simply not been an issue for the twenty years I have been down here, but occasionally you do get hard falls of hail, which can be quite unpleasant.
Your current mileage suggests that you will find it quite a work out at first, but doable once or twice a week to start with IMO. I think its fair to say that 20 miles in Cornwall is twice the effort of that in a flat area like East Anglia, but its not as bad as a mountainous area. The hills are generally quite short but they can be steep, 1:3 (33%) is not unknown and anything up to 1:5 (20%) is common place, and there is not much flat stuff in between so you are spending most of a ride grinding up hill, interspersed with a few short swift descents on the brakes. In distance and height the ups and downs are equal of course, but because you climb so much slower the ups take up most of your time.
I would suggest at least a road triple, only the really fit roadies can cope with compact doubles round here. In fact if I was doing that distance regularly to commute I would get a drop bar touring bike with plenty of gear range, down to around 25" or less, and tyres in the range of 32-35mm: the road surfaces can be pretty dire as well due to all the chip and seal stuff they throw on to the lanes on a regular basis, say once every twenty years or so
. Mudguards are a must for winter IMO and so are very good lights, either hub dynamo or rechargeable of the latest powerful LED type for the front I would suggest.
I can suggest a route or two to try but it rather depends on which bit of Truro you are heading for. Not so familiar with the St Eval end but if you can find a good route to Newlyn East (AKA St Newlyn East), I can guide you in from there.
Avoid the A30 at all costs, between you and Truro it is fast single carriageway road with local loonies doing 70+ on it at commuting times. Commuting at rush hour times in to Truro can be surprisingly busy, and you may find yourself having to filter past long queues, depending which way you go.
I work at Treliske Hospital and only have to do about 5.5 miles each way to commute, but come the summer weather I do quite often go in the long way, it can be truly fabulous in good weather if you get away from the busier roads, which is really quite easy as there is such a vast network of lanes in the county. There is plenty of cycling based activity going on all year round at the weekends, and sunny summer Sundays can be almost over crowded with all the Roadies, Audaxers, End-to-Enders, holiday makers all pedalling up and down the county, then you have all the great off road tracks and trails as well, particularly in my area near Redruth/Camborne, up in the clay pits (St Austell area) and the Camel Trail (Bodmin-Padstow), which is rather too popular TBH. I normally go out with a bunch of friends on a Sunday and clock up 65-75 miles or so.