It is possible to use one of the easy days for interval or speed work e.g. the Tuesday which is only 5 miles. You don't have to do the entire 5 miles of intervals or speed of course. I would do most of these runs using perceived effort and not worry too much about keeping a specific pace. Just make sure I am within a range of effort
Now if you don't have 6 days to train the answer is not to just run faster for longer. You will get injured rather just limit your choice of distance and scale things down and except it will take you longer to get to a where you want. Don't do marathons keep to 5k maybe 10k. You can get decent times on just 4 days at those distances though I have to say I wouldn't do it.
I was speaking specifically about Pauls situation. He is not able to run a lot though he doesn't mention what his weekly milage is. 20% is the minimum. You can go up to 35% of you weekly milage for a long run but that would mean obviously running around 70-80 miles a week and I guessed Paul is not in that situation or he wouldn't have posted.
Your long run should be no more than 20% of you weekly milage done at a slower than regular run pace. The day after you should do a recovery run done at a pace slower than your long run pace or nothing. Any interval training should be no more than 10% of your weekly milage and done after a recovery run day and also followed by one or a complete rest day.
If you lack time, recovery and easy runs and finally long runs are the most important runs you can do as they provide the pysiological conditioning you need to even be able to attempt interval and speed work. I would concentrate on those and leave interval work to every fortnight or just when you can. It will be a slow process but better than crowbarring it in at the detriment of you base fitness.