The logic is straightforward: fifty minutes north on a direct train from King's Cross, flat terrain from station to river, and the most passive transport option available on post-marathon legs: a punting boat steered by someone else. Cambridge does not require you to climb anything, navigate anything complicated, or walk further than you choose to. It requires only that you board a train and sit in it for 50 minutes, which is the correct level of commitment for the morning after 26.2 miles.
Great Northern and Thameslink run direct services from King's Cross to Cambridge throughout the day, roughly every 30 minutes. Advance fares are around £10 to 15 single; walk-up prices are higher. The journey is exactly the right length: long enough to sleep for a portion of it, short enough not to require a packed lunch.
Cambridge station sits about a mile south of the historic centre. A taxi from the station to the river (Magdalene Bridge or Silver Street Bridge) costs about £6 to 8 and takes five minutes. This is the correct decision rather than the virtuous one.
April in Cambridge: The university is in full Easter term, the weeks between Easter and mid-June. The city is working and purposeful, the colleges are largely accessible for visitors (though some college courtyards charge entry in tourist season), and the river paths are in their most agreeable spring condition.
Punting
Chauffeured punting is the non-negotiable activity. Do not rent a self-drive punt. Standing at the stern of a flat-bottomed wooden boat and extracting a ten-foot pole from Cambridge riverbed mud is a task that requires functional core muscles and both legs operating in cooperation, neither of which is guaranteed the day after a marathon. The chauffeur-guided option, which operators including Cambridge Chauffeur Punts (Silver Street Bridge) and Scudamore's (multiple quayside locations) offer throughout the day, involves lying back on cushions while a professional guide poles the boat and explains what you are looking at.
The route covers the Backs, the private riverside gardens of the colleges, passing under the Bridge of Sighs at St John's, the Wren Library at Trinity, and the Mathematical Bridge at Queens' (a wooden bridge built in 1749, rebuilt twice since, and persistently but incorrectly associated with Newton). Approximately 45 to 60 minutes at a pace that allows the architecture to register. Cost around £20 to 25 per person; book in advance for April weekends.
The City
The Fitzwilliam Museum on Trumpington Street (free entry) is the alternative programme for those whose punting appetite has been satisfied or whose budget has been redirected elsewhere. One of the great regional museums in Britain: Egyptian antiquities, European Old Masters, Korean ceramics, early printed books, housed in a Neoclassical building from 1848 whose grand staircase hall alone is worth the visit. The galleries are flat and there are benches throughout.
The covered market on Market Street has been running since 1774, selling fresh produce, clothing, books, and street food in a covered Victorian arcade. Open Monday to Saturday; on a Sunday (the day after the marathon), it is closed. Worth noting in advance.
King's College Chapel on King's Parade is open to visitors in April (entry around £12). The fan vaulting, completed in 1515, is one of the engineering achievements of late medieval England. It is best viewed from a pew, which requires sitting down, which is the correct post-marathon posture.
Where to Eat
Fitzbillies on Trumpington Street has been producing Chelsea buns since 1921. The bun in question, dense, sticky, currant-filled, glazed, is the specific Cambridge baked good and is available warm from the counter. The correct consumption method is immediately, while still in the vicinity of the bakery. The café inside serves proper coffee and a full breakfast menu.
For lunch: The Anchor pub on Silver Street, right on the river at the punting departure point, serves reliable pub food in a setting that allows you to watch the punts from the terrace. The Sunday roast (served until mid-afternoon) is worth arriving for specifically.
Getting Back
Direct trains from Cambridge to King's Cross run every 30 minutes throughout the afternoon and evening. Fifty minutes back to London, then the Tube home. The return is as simple as the outward journey.