Really interested to know how far my spaniels running when we walk/run/bike with him.
can anyone recommend something we can get to work it out?

In my experience it won’t show much difference to your own walk as the GPS won’t pick up lots of small back and forth runs just the general direction they are travelling. Just strap your garmin or whatever to the dog if you still want to try. I did this myself. More interesting was seeing whenre and when the cat went at night.
I suppose you could try a step counter instead.

As you probably guessed someone already makes one for dogs and cats . https://www.whistle.com/
We had one from a market research co that we test products for occasionally , it told us that we have a very lazy cat.