Just take it easy and build up. I too prefer to start them at 18 months ish, but minimum 12. I would make sure I knew the dogs history though (parents hip scores etc).
Start off on the lead on nice easy routes or if the dog heals well try and get it to ‘wheel’ ie sit just behind the back wheel.
Generally on the trails I ride I let her go in front, she is much quicker at responding to rock slips, fallen trees etc and generally it is when I think I know best and choose another line that I end up having a bad crash and end up hanging over a barbed wire fence.
Just make sure you can trust your dog, get it to heal/wheel when needed, recalls promptly and take lots of breaks and don’t overdo it as you will cripple the dog. Next time I would go with something with longer legs than a spaniel for the types of rides I like to do.
Oh and always have a plan in case one of you gets injured – ie someone who can pick up the dog.
a gun dog (proper one, not a show one and with good background) should be able to go all day eventually but with plenty of breaks and choosing the terrain right. If going for a more intensive ride obviously lower the duration. I sometimes arrange to meet the OH for an ice cream at a tourist spot and she picks up the dog and then I carry on.