I'll try giving an answer to why you in particular should not travel to that isolated spot where nobody else is and go for a 3 hour walkk there.
On this thread the government are being blamed for not being precise enough in their instructions, they should offer a clear blanket rule not open to people's interpretations, because otherwise people decide that what they're doing is really ok.
For example - don't go outside, but you can for exercise - how often is the right amount? how do you exercise? where do you exercise? when I've finished can i sit in the park for bit with my mates?
So, in the example of your 30 minutes car ride to a secluded spot where there's no one there and you walk for 3 hours.
How would you expect the government to make a clear rule about what is and isn't allowed? Guaranteed you'd have people saying well if he can go for a 30 minute drive, surely I can drive down to the coast? If he can go to that secluded spot where there's no-one, surely I can go to that local walk where there'll only be half a dozen people in miles of space, or to the beach because there aren't supposed to be many people out? In fact if I only stay for a few minutes I can probably meet up there with my mates. And suddenly everyone is driving around where ever they like.
It happened in Sydney. Stay home, but you can go out for necessary exercise. Last beautiful day of summer and thousands of people in Sydney flocked to the beach, stacked up side by side. So the government had to say, no one goes to the beach anymore. People are stupid.
You want to know how stupid? That beach closure was a week ago. On tonight's news some lunatic with his two kids next to him was interviewed saying: "It's unfair that me and my kids can go out on the water together in our canoes. We live in the same house. Coronavirus can't be transmitted on water." As soon as you say yes to that, you open it up to every surfer ... every boogie board rider ... every body surfer ... every swimmer ... every partner of said persons ... their children (are you going to stop children wading in the water while their dad is out on his jet ski?) ... every wader ... every paddler ... every dog walker ... etc, etc.
Just do what the government is saying.