Length: 18¾ km (11.7 miles), or 16½ km (10.3 miles) if finishing in the village. Toughness: 5 or 4/10
For the cheapest (Thameslink-only) fare from central London, take the 09:00 Brighton train from St Pancras (Blackfriars 09:09, London Bridge 09:15), arriving Haywards Heath at 09:59.
Alternatively, take the 09:24 Eastbourne train from Victoria (Clapham Jct 09:31, East Croydon 09:40), arriving at 10:07. [From Victoria you could also take the Gatwick Express service at 09:29; this calls at Gatwick only and arrives at 10:11.]
At Haywards Heath make your way to the Perrymount Road bus stop for the East Grinstead-bound Metrobus 270, departing at 10:21 for the 15-minute journey to Horsted Keynes Station. Bus fares are still capped at £2 each way.
Buses from Horsted Keynes village back to Haywards Heath are hourly at xx:39 until 16:39, then at 17:57 and 19:34. All except the last one call at Horsted Keynes station four minutes later if you choose to complete a full circular walk. At Haywards Heath there are four Thameslink trains an hour back to London (at xx:01, xx:19, xx:31 & xx:49) plus six Southern ones.
An early train with a bus connection doesn't suit everyone, but I've chosen this somewhat neglected walk because there's a Bluebell on Parade weekend on the heritage railway. I've chosen to start the walk from the Bluebell Railway station rather than the village because this way you should get a close-up encounter with a couple of steam trains on the stretch alongside the track, a southbound one just before 11am and a northbound one shortly afterwards.
After an undulating climb onto the fringes of Ashdown Forest you should reach the Red Lion in Chelwood Gate at around 12:30-45. There's an optional short cut at the start of the afternoon leg, which might tempt you as the gorse and heather won't be at their most colourful in June. A gradual descent through woods and fields brings you back to Horsted Keynes, where a couple of decent pubs will be vying for your custom before the bus ride back to Haywards Heath.
You'll need to bring the directions from the L=swc.132.a page.