Well I'm doing intervals once a week for my HM training (Windsor in May). I have mixed feelings about intervals: they are MEAN and they are NO fun but I can see my speed picking up overall.
I'm only doing them now because I'm already trained for the distance, having done a HM last week. I did not train for distance and speed all at once. (Hurty). With my second HM I am aiming to get under 2hrs (from a pesky 2hrs 2mins). Been running seriously about 5 years, aged 40, want to be running when I'm 70, NEVER doing a marathon...)
Every third week I do hill repeats instead of intervals (yey). I also do a tempo run (4-6 miles), a long run (8-12 miles) and a little recovery run (gossiping with a friend) each week, so I've taken all the advice and I think it's pretty balanced. Apart from the cross-training. Can't be arsed with cross-training (lazy bad girl).
I would advise that you listen to your body. I think experience tells you the difference between "tired because I'm getting fitter" and "tired because I'm wearing my body down too much".
I do find the intervals tire me out for my longer runs but I need to slow them down anyway. So I'll keep going for now. If anything's going to give me injuries though it's the intervals. I tried them in last year's training, found that they wiped me out too much, so I dropped them. Feel better this year, but again, intervals are taking me to the edge of my energy. And I will drop em like hot cakes if I feel it's going too far.
I think there's a halfway house though, (which I might go for if intervals and me don't get on again) - perhaps you could just carry on with your normal runs and add a small speed element to one or two of them. A minute's sprint at the end of a long run. A couple of 5-min blocks of extra effort here and there (fartlekky fun). Hills are also a form of speed work don't forget - your legs won't know the difference.
No matter how fit you are before you start running, it's fitness-for-running you need to build. I have been AMAZED at how my body keeps adapting. Every year I feel better and better and stronger and stronger - and I'm the other side of 40! Running has taught me so many life lessons and taking the 'long view' in life is one of the most important.
Nothing wrong with staying with your favourite shoe Naomi. I've had pair after pair of Asics 2170s for 3 years and I'm sticking with 'em. Really surprised that your new pair of Omnis was so different though. That must be really disconcerting and I'm not surprised you're wondering where to go from here. Have you tried returning them and getting another Omni 8? - you could have had a dodgy pair that fell of the production line? it happens.
However long-term, the gait analysis IS the way to go. Though combine it with your own instincts. My local running store offers it but like yours only stocks a few brands (luckliy Asics is one of them). If I were you I would give myself a massive day out to your nearest big running store. (I could spend all day in mine). Someone will watch you on the treadmill and show you the video so you can see what's happening. You will then try on dozens of possibly suitable pairs, have a little trot in each and choose your favourite.
You might be surprised too - I (and my gait analysis guy) was shocked to see that I do NOT overpronate despite having the flattest feet in the human world. I could go for neutral shoes but the stability of the Asics 2170s seems to suit me and they're not doing me any harm. Last time I went for slightly different Asics Gel DS, as they are a lighter version of the 2170s for speed/races. These give slightly less support and my calves have had to strengthen a bit but all good now. I alternate between running in the DSs and the 2170s and just update them when they're knackered (when I get a random surprise new niggle somewhere I know it's new trainers time). I am tempted by some of the pretty Adidas ones and Nikes look cool but if it ain't broke... I tried out some Brooks just out of interest as a friend loves them but after 2 miles I cried all the way home... When you know, you just know.
"Zitty head"!? ... an image I'd rather not have in my mind....
Tried a snowy run today tho gone slushy here, it was fine once I let go of my target pace and turned it into a fartlek run. Speed up on the clear bits recover on the icy bits. Extra aches tonight tho, it must change your gait, tenses you up I suppose. No falls thank the lord. 5 weeks til HM - trying not to lose touch with the training plan but refuse to go deadmill... but it is a great experience to be out there in the snow.