Have you thought about the line from Leighton Buzzard to Welwyn, which you could drive to in minutes. I'm not sure about the first part from Leighton Buzzard, I suspect that a lot of it has been lost to the A505, but from Stanbridgeford to Dunstable North (by the A5) it is open as a cycleway. I'm not sure about the section through Dunstable but it is tarmacced from near Dunstable Town station (A505 bridge) and possibly before that, it's the furthest west that I have gone) to Chaul End Road in Luton, the path runs alongside the guided busway. You can then pick it up again just beyond Luton Airport Parkway station, the entrance is off the B653 just after you pass under the A1081 Airport Way. It runs virtually uninterrupted, despite being built over at Harpenden East Station, to Leasey Bridge Lane level crossing, Wheathampstead although the last mile or so is untarmacced. From there to Wheathampstead you can see the line, and where the goods yard was, but there is no access to the trackbed. Visit the platform at Wheathampstead station by the B651/B653 roundabout. You can then access the trackbed from the B653 about half a mile from about half a mile from Wheathampstead station and follow it to the A1(M), pick it up om the far side and you can follow it all the way to the Network Rail boundary fence. If you've still got enough energy having got that far, do the Welwyn - Hertford line.
I've also walked the Brampton Valley Way where it runs alongside the Northampton and Lamport railway and would recommend at least that part, lots to see. Also I'd second the suggestion to do the St Albans to Hatfield line. Its all paved and the platforms are intact with information boards at some if not all.