Everyone has a different level of basic fitness. So if you haven’t been running the first week will vary depending on how much other activity you usually do. It may not even be running the first week. The reality is that if you can’t walk to the end of the block without breathing heavy, you probably shouldn’t start by trying to run a mile. If you play soccer twice a week and ride your bike 20 miles to work each way, you are going to be great starting with a mile.
The first week running is a challenge and where most people will drop out of a running program. The reasons to quit are many, it takes too much time, it is too hard, or my personal favorite it hurts too much. If you start running with a large amount of running (for you) it is going to hurt. My first week I started running 5 minutes. I hurt a lot. Now even if I haven’t run for a couple of weeks, I can go run a couple of miles and hurt less. The best way to start is to do just a little bit more than you do now.
So the first day walk fast for a few minutes to warm up. If you find walking fast leaves you completely out of breath, that is probably going to be your max for the first week or two. Alternate walking fast for 2 minutes and walking normal for 2 minutes to catch your breath. That’s it. Try for 10 minutes to start, but even if you can only do 1 x 2 minutes that is a start. The next morning will be a good way to judge if you have overdone it. Are you a little sore or are your muscles tight? That is what you are looking for. No sharp pains, those are bad, and some pain may occur the first few days especially.
One of the biggest differences I suggest is to walk every day if you cannot run. Keep adding a section of fast walking every couple of days until you have built up to 12 sessions of fast walking which should put you at close to a mile including the slow walking between the fast walking sessions. This may take several weeks and that’s OK. If you are looking to do a 5k, you could probably start a 5k training program at this point. The official couch to 5k program is a good place to start.