As long as you don't have medical problems, you can use the cardiovascular recommendations on line for cardiovascular work out which is related to age and pulse (heart rate) or look for guidelines on building strength versus muscle mass. all have different guidelines, and you don't indicate your specific goal.
I find it best to find my own pace. A speed where I feel comfortable breathing and don't get any cramps or aches. If you have a hard time keeping up with the breathing you are probably going too fast. I recommend a trial and error approach where you try different speeds and find one that you feel you can handle steadily without having to stop or slow down for at least 20 minutes. Eventually you will work yourself up to faster and more demanding speeds as your endurance improves. It also helps me a lot to do a one kilometer brisk walk warm-up. Hope this helps.