Shoe angst - help!

I'm currently running, very comfortably, in an ancient pair of Nike Zoom Airs. There's about 300 miles on them (very old but not worn out due to my just returning from a 7 year break from running, having bought these just before I stopped last time). If it helps, I've got a neutral gait and a midfoot strike.

So I went to Runner's Need, who analysed my gait, and sold me a pair of Gel Nimbus 13s - Asics. Tried a regular 3 - 4 miler in them and had to stop running after two miles with severe shin pain (I *never* get pain in my shins when I run). The shoes were far too padded and it felt like I was running on two weighty, overstuffed sofas which dug into my arches, horrible. Sadly I was told I can't take them back. This was a few weeks ago - I tried a two mile recovery run in them this morning just to make sure it wasn't me wussing out last time, and same problem. So it was back to the Nikes.

A few weeks after buying the Asics, I went to Run and Become and shared my trainer woes and preferences in a bit more detail. I tried on, and loved, a pair of Inov8 225s, which feel much lighter and more comfortable. I'm really looking forward to these becoming my regular shoes.

However, I'm training for a half marathon at the end of October and am worried about trying to transition and train for my first half. I was using them for my 2 miler (am running 3 - 2 - 3 - 6 at the moment, and plan on increasing the first, third and Sunday runs in line with the Hal Higdon plan) but having now read Inov8's website, they recommend you only run for 5 mins at a time, building up slowly over 16 weeks. When I ran a 2 miler then a 3 miler in them I found my right calf and hamstring and knee playing up. I reckon I will eventually get used to them though.

So I need some advice from wiser runners than me! I'm not planning on running in the Asics again so any suggested future uses for them would we welcome. Regarding my main trainer problem, should I:

a) carry on in my slightly knackered old trainers and transition to Inov8s after the half marathon; or

b) transition now and risk injury or training in a weird way which might mess my times up; or

c) buy yet another pair of shoes like my old ones and switch training between old and new, then transition post-race?

Am confused And didn't realise training was going to be quite this expensive!! Would really, really welcome some help on this as I'm banging my head against a brick wall and don't really know enough to stop myself getting injured. If I *have* to shell out on a new third pair of shoes I will but would prefer not to have to.