Nice one Ergates, i myself have started commuting again by bike after a few years break. Though nowhere near your distance. Just by making that effort to get to work shows you are up for it.
Within a week i have already felt a difference, i think the best plan to to just enjoy it. Luckily i have picked a slightly longer route route that goes through a bit of countryside, so it isnt't hard. I do a bit of hiking, but the different muscles used for cycling are hurting a bit at first.
I agree with the idea of keeping on using your present bike. With a comfy saddle, good road tyres and upright riding position, a mtb can be quite good for the daily commute. I am using a cheap
ebay buy with road tyres on and will keep on using it for a few months before deciding to maybe change to a proper road bike. I guess if we make it harder for ourselves now, on a sleek road bike we will be flying
. I have seen a lot of people go out and buy the flashiest bike going and then give up within a couple of weeks because they weren't comfy.
I also make sure i get a good breakfast of whole porridge oats and fruit etc for the slow burn carbs etc. And not forgetting a drink of water before hand.
All the best, Craig.