Define the comfortable core route together
Ask the person with the shortest comfortable range what would make the outing enjoyable. For example, a ten-minute walk to a café, seating there, and a ten-minute walk back may be workable. A twenty-minute one-way trip is a different plan even though the number on the clock looks similar.
Draw the return journey, not just the outward leg
Pick an entrance with a known path to the café and confirm suitable seating at the proposed time. If others want a longer route, choose a loop that returns to that same café before the group must leave. Avoid asking anyone to wait alone unless they actually want to.
Check conditions on the day
A park loop might look short on a map but have no convenient exit halfway around. Check entrances, seating, and current access information with the venue. If the information is uncertain, pick a route that lets the group turn back easily.
If a lift is unavailable or a usual entrance is closed, the change may affect the return route as well. Agree that anyone can shorten the walk without renegotiating the whole day. Keep a nearby activity or seated meeting point that works if the optional extension no longer fits.