Hello @Doubleglazing84
Thanks for posting about this. The Strava App is not actually designed to save activities in phone memory and this behavior may be unpredictable if you try to use the App in this manner. For reliable results, we recommend saving and uploading after each activity. We also don't recommend trying to record a single activity that is more than one day in length.
There are many third party GPS devices that can reliably record and save multiple activities on the device until you get a chance to upload; that might be an option you want to consider. Many of those devices, including popular ones like Garmin and Wahoo, can then sync that data to Strava.
Another option would be to enter your activity data manually into Strava.
I hope that's helpful. Best wishes for your hike and we would love to hear an update about how it goes.
Thank you, I discovered my Coros Apex Pro has a Hike program that allows you to do multiple day hikes whereby you can upload when you have cell service. So this should work well.
Many thanks