Another 12in12er here - I'm doing 12 throughout the year. I had planned one per month, but since I was woefully under-trained for Gloucester (scraped in at just under 5 hours) - I'm taking Feb to regroup so will have to double up later in the year.
Planned races now are:
Mar - Duchy (Redruth), Apr - Shakespeare (Stratford), May - Brathay (Windermere), Jun - Wales (Tenby), Jul - Much Wenlock, Aug - Salisbury 5-4-3-2-1, Sep - Wolverhampton and/or Robin Hood or New Forest (?), Oct - Abingdon, Nov - Rutland, Dec - Portsmouth.
Unlike a bloke I saw at Gloucester, I'm not going to make a t-shirt just yet as this list is subject to change (his had him down for Cambridge in Feb which I wanted to do too, until I found out it was in March and clashes with the Cornwall one already booked).
Another possibility, which I've yet to research fully, is a summer one in Kent with 40 laps of a loop (just over 1k). Unique among marathons in that it has 40 feed stations to keep you going (and your own personal lap counter). Mind numbingly dull, or an interesting alternative? I'm undecided on this one!