What Richard 2 said. You could start by adding in one session a week, it could be a short session and you could shorten your longer session to compensate.
As to the intervals. The aim is to run, so I'd go for eliminating the walk section. Try to moderate pace in the run section so that you don't need as much time to recover in the walk, jog really slowly.
I think that once a week will allow too much time to detrain between sessions. Ten minutes three times a week would be better than thirty minutes once a week.