Unfortunately I suspect you'll be starting too early for me - Guildford is an hour and three quarters from Hove by train (via Havant!), and £20. Horsham and meet you en route could be feasible I suppose.
38 miles of flattish trail doesn't sound all that much, but when I rode this trail in mid-winter, not much of it was fast, and quite a lot was heavy going. Three of us met up in Horsham, did some Sustrans-related recce work around Horsham, followed by the run back to Shoreham, so I suppose approaching 30 miles in all, and we reached Shoreham just before dusk with no pub stops. Doing the full 38 miles twice in a day in mid-winter would be a major epic IMO.
What time will you be leaving Guildford?