I want to walk like 10 km with a pace of 11:00 minutes per km and keep track of my pace using Virtual partner. But I can't set the pace higher than 8:50, how can this be ?
I want to walk like 10 km with a pace of 11:00 minutes per km and keep track of my pace using Virtual partner. But I can't set the pace higher than 8:50, how can this be ?
But I can't set the pace higher than 8:50, how can this be ?
Probably because it was originally designed for running (and perhaps for the speed walking). Not a lot of people races by slow casual walking 
You can use low and high pace alerts instead.
All best for your race! As I wrote, you can use the pace alerts on the watch for this purpose, and can add lap- & overall- pace fields to the Walk activity screens, so that you can see whether you are within your planned pace since the beginning as well as during the last lap. You can set the auto-lap to whatever distance you want - 1km, 5km, 10km,... or tag the laps manually as well.
And you can also use the Workout feature - creating a workout for 100 km (99.99 is actually the maximum I managed to enter when testing it) and the desired pace range. For the Walk activity type, you have to select the Workout type "Custom".
Once you start the workout from your Walk activity menu » Training » Workouts » your workout » Do Workout, there will be an additional screen with a graphical pace indicator showing you whether you are within the planned pace range. See a screenshot from my Instinct (it may look better on your Fenix):
And yet another option is to use a Connect IQ app - in the CIQ store there are plenty of apps for pace, that may be applicable for your needs. I did not test any, but I am sure some of them could work for you.
You'll need to test the impact of the CIQ app on the battery, though, before the race. Some of them may increase the consumption. The battery capacity will be certainly limiting anyway, even without any app - although the specification claims up to 24 hours in GPS mode, I would not rely on it. First of all, the specs are always based on some idealistic lab conditions, and then at an old device like the F5, the battery capacity will be likely already rather reduced.
You could also use the UltraTrac mode with (theoretically) up to 75 hours of battery life, but test it first, because on many models this tracking mode is rather useless - often most of the data (including the track) is completely wrong.
Good luck with your challenge!