Hi, I too used to live in Finchley and moved to Hertfordshire when my DS was 18 months old (he's now 3). I now have a 5 month old DD too.
I honestly think there's just as much here, if not more, than in Finchley. I agree about the walking aspect - I walked everywhere with my DS and like you, lost my baby weight relatively easily. I think the nature of being in a much smaller town outside London is that more people do use their cars and there is less of a public transport/walking culture. Having said that, there's some great local walks and once you've met like minded mums you may find some people to join you on walks - I know I have and regularly meet locally with a mum I met at playgroup to go on walks. Or just a wander around one of the towns you've mentioned passes the time and helps with that weight loss!
I think once you're off work and truly looking for mums to socialise with, you'll have no problems. I attended an NCT toddler group in Hitchin, held at the Hitchin Youth Trust and met some great mums there. You don't have to be an nct member and many brought along little babies too. There are lots of playgroups in Hitchin (netmums lists them), and people are really friendly. There's also a great rhyme time at the library, drop in swimming sessions at the pool and socials associated with playgroups (Hitchin Mama has a reading group, cinema social group and regularly have a table at a local pub quiz. Mama meets on Fridays at Christchurch on Bedford Road).
I know St Albans also has a good library with rhyme time/story time, and there's a lot of mum/baby/toddler groups. If you're willing to travel that way, you might also want to check out Harpenden - lots going on there (look at mums guide to Harpenden online).
I don't know so much about WGC but the theatre there is great for kids shows and they have also recently opened a soft play there. I'm sure there must be plenty of other things going on - often things are advertised in cafes.
One other thing for mums is Letchworth and Hertford cinemas: both do parent and baby viewings. Oh and most villages have good communities running playgroups in village halls.
This is really long, sorry! But in a nutshell, don't panic! Once you start looking and going along to things you'll find there's loads to do and it's a great place to be a mum. PM me for any other info - or if you wanted to start your local mum socialising and wanted to meet up for coffee!