A lot depends on how much you are willing to spend, and what sort of running you are wanting to do on it. In my experience cheap treadmills are not worth the money (sub £200), unless you are just looking to use them for walking on.
If you have £500-1000 to spend on one then you are starting to get into the more useful bracket. Personally I've got a Horizon Elite 507 (had it for 6 years) and would recommend it, it has a good powerful motor, goes to 12mph and incline to 12%. Would also recommend fitness-superstore.co.uk, that's where we got our treadmill from and they were helpful and efficient.
Again speaking personally what has made a difference for me to get from 44m to just above 40 (haven't cracked the 40 yet, but now definitely feel that I could) is introducing some speed work sessions. These have taken a mixture of track sessions 10-12 400m repeats at 90secs or slightly quicker with a 100m walking recovery, or a loop of about 700m on the road with a 60-75 sec walking recovery repeated 6-8 times.
What you could also do is incorporate some speed work into your regular paced run, say going for 1 or 2 Km at your desired speed to break the 45min barrier, so that would be something like 4m25 for a kilometre.
I also made sure that I was regularly doing runs over the 10k distance so a 'normal' run would be 7-8.5 miles, I didn't get any time to do even longer slower runs but that also seems to help as well, as a lot of it is about how much time you've spent on your feet...
Horizon Elite 507 gets my vote - Had one for 6 years, good piece of kit. The company completely replaced the machine 6 months ago as the incline motor separated from the frame - thank goodness for the lifetime frame and motor guarantee. The company was very good in dealing with this and I had a replacement in no time, otherwise no problems with this machine. It cost £999 when I bought it, not sure what it is now, got it from Fitness Superstore online, they also were good to deal with. Just my 2 cents input for what it's worth.