Hi
@Tharina - I'm assuming by not using an "online" device, you mean something not using Internet / data.
There are lots of apps that will measure distance (in km / mi, as you prefer) without requiring online access. Your smartphone does not need online access to determine your position -- the GPS antenna can do that and GPS does not use data. An app can record your positions and then use simple calculations to sum up the distance between the points.
There are many apps you could use, including ones that will let you export a GPS track of your walk, send daily announcements to social media, and so on. One thing to be cautious about, though, is that the GPS antenna uses a lot of battery, especially if it is running continuously (as would be the case when recording a long-distance walk). Well-designed apps try to minimize this by reducing the frequency that they check your position, but it can still be an issue when you're walking 4 - 8 hours per day.
Another possible issue is accuracy. GPS has an inherent margin of error. Your device also makes a big difference. The GPS receiver in a smartphone is not as accurate as a dedicated GPS. If you really want to know if that section of the walk was 6.2 km or if it was 6.3 km, then a dedicated GPS receiver would be best. Of course, if you want to know whether your walk was 6.216184 km, then the suggestion by
@Viranani that you use one of those gizmos with little wheels to physically measure the distance is your next step (or a really long measuring tape).