What’s a good swimming routine for Middle Aged guy getting in to swimming?

I had swimming lessons as a kid and teen so I have done all the main strokes before. I’m taking up swimming because of a leg injury and my doc advised me to swim. The leg is mostly healed now. I can bike and walk and drive. 

What is a good way to structure swim work outs? I started this week reserving swimming time at a pool. I can book an hour at a time. I usually swim up and back twice, rest, do some water walking and then go up and back twice again. Or I just rest after 2 laps and then go for two laps again. I need to lose weight. 

I like to make swimming part of my routine now. I used to just run on the tread mill. Now I’d like to alternate between swimming and biking. What is a good swim work out please? Thank you!