I usually take our dog out for about 45 minutes-1 hour in the morning, and my husband takes him out in the evening. If I were on my own, I would continue to do two, shorter, walks. I see people jogging and cycling and there is no knowing if they have been out and about for half an hour or three hours. I think a reasonable amount of time is one hour.
Expert opinion seems to be that, provided people keep at least a 6 foot distance from each other, and preferably more, there is little chance of contracting the virus. In my experience, many people cross the road if they see someone walking towards them - and I cross over if they don't.
Going to areas where there are large numbers of people - as happened a few days ago when there were crowds on beaches, in parks, etc., all fairly close to each other - is a different thing because it is difficult to maintain the required distance.
We are allowed to have some sort of exercise - eg walking, jogging, cycling - once a day. It is important that people who are not knowingly suffering from the virus get exercise, particularly older people.
In reality, I think the vast majority of people will eventually get this virus as it is nigh on impossible to maintain a completely sterile environment for an extended period of time. As I understand it, the government is working on that principle anyway but is trying to "stagger" the incidence of the virus so that there isn't a huge number of people at one time needing emergency treatment.