The Barleywolds are great cycling country, with quiet roads, rolling countryside, beautiful villages and ancient pubs. It is easily accessible from the train stations of Royston, Ashwell & Morden, Baldock, Stevenage, Watton at Stone, Ware, Bishop's Stortford, Stansted Mountfitchet, Elsenham, Newport or Audley End. Almost all roads are cycle friendly, except the north-south A10 and the west-east A507 and B1038. So just dial in your starting point into your favourite cycle-route planner, and specify a round-trip to somewhere in the Barleywolds. An example is given below, from: https://cycle.travel/
To cycle from Hackney in London to Buntingford via the Lee Valley would take 3 hours non-stop, so you would need to take a train at least one way.
To cycle from Cambridge to Chrishall would take 1 hour 15 mins, so possible to make a round trip, or take train back.
If taking bike on train, for the Kings Cross London to Cambridge train, get out at Baldock, Ashwell & Morden or Royston station. On the Liverpool Street London to Cambridge train get out at Stansted Mountfitchet, Elsenham or Audley End. This train journey should take about 60 mins.
In the map below, the blue/grey area is the Barleywolds, and the blue icons are nearby train stations from which the area can be accessed by bike.
Click the + or - icon at bottom left of map to change scale and explore map.
Baldock Circular Ride is 16 miles long, lasts 3 hours., through Weston, Clothall and Wallington. It's away from main roads, but there are some steep hills.
https://tarmacs.app/cycling-routes/united-kingdom/bedfordshire-and-hertfordshire/walkern
https://www.komoot.com/guide/775529/cycling-around-buntingford
Try the Buntingford BMX Pumptrack at Norfolk Road Playing fields, Bowling Green Ln, Buntingford SG9 9FQ