The plan is to walk from Tadworth to Merstham, around 7 miles.
Starting at altitude 233m/764ft and ending up at 96m/315ft this can
honestly qualify as a gravity assisted downhill walk. {ignoring any
minor bumps on the way}

Train to Tadworth from East Croydon. 10:13 from East Croydon, 10:15 from
South Croydon, 10:18 from Purley Oaks. Change at Purley for the 10:28,
arriving Tadworth 10:48. All inside TFL zones.

Trains home from Merstham are two an hour but only 13 minutes apart.
Direct trains take 14(!) minutes at 30 minutes past. Trains with a
change take 21 minutes at 43 minutes past the hour. Out of TFL ticket
needed. £3.80 or less with a railcard to Coulsdon South, back in TFL
territory. Also useful info from Dionne, the 405 bus is another option
from Merstham to Croydon, hourly at 26 past, so lots of workable routes
back home. Currently no stoppages announced for these services.

The walk covers Banstead Heath, pub stop option in Mogador, which might
be a little early, over the M25 to Colley Hill, along the North Downs
Way East to Reigate Hill, Gatton Park and Pilgrims Way to Merstham. Map
attached.

There is a cafe in the Reigate Hill car park. Apart from this, limited
over the counter refreshments. Bring a picnic and drinks, always a good
plan. There is a pub in Merstham that we could promise ourselves a drink
in at the end of the walk, time permitting.