Long run hip pain

Hi all,
I've been running on and off over past few years, at 5k distance levels, and not very consistently, but this year got back into running and decided to follow a half-marathon training plan, via Zwift on the treadmill. I'm 12 weeks in, and weekly long runs are now up to 18km, and starting to take it's toll on my right hip.
I'm 40yrs old, 5'9, 88kg(14st). Ankles, knees otherwise are all fine. Not feeling any other issues from the training so far. Left hip is fine too.
The hip has come to my attention as the distance has grown over 10k, but has not been something I'd noticed before. No history with the hip, no other issues in general.

I did 16km on Monday, at ~6:30/km pace. (My 5k pace is around 5:30/km for reference. The Zwift plan builds in 5k tests and adjusts pace for the tempo / speed / long runs in the plan. So the pace feels manageable, and has gradually built.)
But my right hip was definitely starting to hurt towards the end. By the evening, it was pretty sore. Stiff next day (Tues) so I took a rest day, this morning (Wed) it felt good again so I did 15km on stationary bike to keep things moving.

I have previously run early in the mornings, while fasting, but for longer runs, I'm doing them later in the day where I can get a little bit of fuel onboard beforehand, and easier to get a meal in afterwards if needs be.

I'm not intersted in hurting myself or doing any damage, but realise I probably need to expect a little bit of pain while I start to extend the distance.
So where's the balance? At what point do I stop and ease off?

Any advice welcome please!

Tom