Runa

Open the file is the whole instruction. A download that succeeded tells you nothing about what is in it.

Do the export properly now, before the deadline, and treat the file you already have as a first attempt rather than the answer.

Check what is actually in it. Exports vary enormously, some give you every raw sample, some give you daily summaries, and some give you a subset with the interesting parts missing. Open it, find one day you remember, and see whether the detail is there.

Look for a second export route. Many services have both an in-app export and a separate account-data request, and the second is often far more complete. Also check whether there is an API you can pull from while the service is still running.

Export more than once, and keep the raw files untouched somewhere safe. You can always parse them later; you cannot re-export after the shutdown.

Check whether it also synced anywhere else. A lot of these push a copy into a platform health service or a third party training site. If it did, that copy is yours already and may be more useful.

On making sense of the file: for most of these formats there are open source tools that convert them into something readable. Worth searching for the specific service name plus export, you are very unlikely to be the first person doing this in the month a service announces its closure.