So, I'm not all about running..
I like bouldering too, and actually find it really helpful for getting a bit stronger, building up my core muscles, making me (hopefully) a bit tougher on the trail!
I have my favourite places to boulder indoors to (if I don't venture outside to boulder). One being Rock Over in Manchester, which, when the weather is less than optimal enables you to climb some good problems in a spacious environment (plus get a good cuppa when you need a break).
The other, for when I am in London, is Vauxwall climbing. Nothing is more reassuring than the rumble of the tubes overhead (it is based in the arches underneath the train lines - so very accessible by London Underground!). The staff are really helpful and friendly; it's also a good place to actually go and work if you need a remote work/train kind of day (has good wifi), and three climbing rooms, plus a weights/work out area.
Bouldering FAQs: a quick check
What do I need to start bouldering?
Rock shoes + chalk bag. Both hireable if you don't own your own. It helps to wear comfortable clothing you can move easily in. I opt for leggings and t-shirt/tank normally.
Is it very high? What if I fall - do I need ropes?
Indoor wall bouldering doesn't require ropes - hence being bouldering rather than route climbing. It is not massively high at the wall, and there is a soft crash mat landing for jumping off. There is an art to landing so you need to treat bouldering with care and caution, but when approached properly it is a safe sport - like any other.
Is it only for the WAD climbers and super fit?
No, it's for everyone. A great way to get strong - problems are graded according to difficulty so you can challenge yourself appropriately. It is also a fairly cheap sport - pay for your session (and shoes + chalk if you don't have your own.) And just go.
The British Mountaineering Council has all the know to do with indoor climbing, do visit their page on indoor climbing