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Should I just get on the bike more often and HTFU?
I'd give it some time. Obviously stop if it gets progressively worse or seriously painful but, as others have said, it will just take a while for the core muscles that help maintain your position to come back.
If you're not doing regular decent distance rides then you could also do some core strength exercises off the bike, should only be 20-30 minutes a few times a week. Don't have any specific recommendations, others may have, I'd just resort to google.
I cycled a decent distance yesterday for the first time in ages. My legs are tired today which seems fair enough. My arms are also tired which I think means my bike position is all wrong? Is there a bike fit thread? Should I book an appointment? Who is good (central or north London)? Should I just get on the bike more often and HTFU?