20 miler in March, would give you 3 1/2 months until Cotswold so should be fine. Training plans will have you running 20 milers, so a good test. Anyway, not much fun putting all your eggs into one basket and not really enjoying the build up.
Ticking along nicely. Swimming is OK and lolling to start increase the frequency. Biking going well, high RPM sessions on the gym bike, and good road rides on the singlespeed. Running OK, 20k is longest to date.
Hoping to cycle the course in late March just to get to know the course, and then focus the training to suit.