Unless you know exactly what you're after, the expertise of most shops is worth paying the extra in comparison to online prices. Paying for an anlysis is a bit much though when there are plenty who'll go to the ends of the earth to help with anything you could wish. At worst it should be free if you make some kind of purchase.
Your fitness will still be significantly better than the start of the year so congratulations on that and achieving what is likely to have been one of your goals when entering the marathon. The thing that will hold you back on race day is to the extent your hurt is injury. A sports injury specialist will be able to give you a diagnosis on whether you should run (not a GP with little sports knowledge who will almost certainly err on the side of caution)
Hi Alan, it's not surprising that training for a target that's months away can lead to fluctuations in motivation. Is there a parkrun near you? Being able to regularly see that you're making gains is good for motivation - http://www.marathonrun.co.uk/marathon-training-advice/the-seven-wonders-of-parkrun/
You may also find dropping to four runs a week, but incorporating more variety in terms of distance and speed makes your runs more interesting and offers better stimulation while also allowing more rest. Just a suggestion to think about and I concur with roball about getting involved with a club where you'll get plenty more advice that you can tailor to suit you accordingly.
Hi Lottee, you'll be surprised what gains you can make by simply keeping up the consistency and a massive part of that is doing the kind of running you enjoy, be it a certain terrain, as a social element etc. You can take 5 or 6 minutes off that 5k time without worrying too much about particular sessions as if you've recently started again, your aerobic fitness will improve by regular running and slowly increasing your weekly mileage. When you begin to plateau, that's when speed work builds on the aerobic base.
I'm not suggesting to not do faster sessions if anyone in this situation wants to do or enjoys doing them, but emphasising that they're not a pre-requisite for making significant improvements at this stage of training.