There's an ever growing array of half marathons on offer. These are long tough distances for anyone and particularly if you don't have some years of training under your belt. So here are a few selections within the UK that all have a reputation of being well organised. I've put in bold the 10 events that many people rate as at the top of the pile. I've also put in italics the 6 races that are the most undulating. Of course, feel free to cast your net wider, and indeed if you are outside England you'll probably have to, but if you want a strong element of quality assurance these should keep you focused at any stage of the year. Dates are usually the same weekend each year although the early spring races may shift around depending on the dates of Easter and the London Marathon. Also check out races at Dorney Lake hosted by various race companies, and the runthrough race lists (note not all their races are certified as to correct distance).
It's maybe nasty to put a spoiler in here but there's one very large event in London that seems to get serially poor reviews so it's maybe not one to beat yourself up about entering (no further clues).
Brass Monkey (Yorkshire)
Roding Valley (Essex)
Tonbridge Wells, kent
Paddock Wood, Kent
Ranelagh (Richmond, SW London)
Robin Hood, Nottingham
Great Eastern Run, Peterborough
Cabbage Patch 10 miles, Twickenham (Note 10 miles not 13.1!)