Many of us are time poor. Too much to do with too little time. If you are keen on improving your running performance in any event from 5km to marathons, then I recommend one workout above all others. The workout that I believe can reap the most benefits when done once a week is the Tempo Run, also known as the anaerobic threshold run.
The tempo run is a steady state run at a hard pace. It is run over any distance from 5km-18km, depending what event the athlete is training for. In any training period, 5km runners up to marathon runners can incorporate a tempo run as part of their weeekly schedule. Unlike interval sessions, there is no break allowing you to recover from the hard running. You run it at your anaerobic threshold for it's duration. If you have ever raced in a half mararthon, tempo pace is very close to your half marathon race pace. Runners will move their lactate (the by-product of anaerobic respiration) threshold further and further, and the pace to get to your threshold gets higher. I like my runners to finish the session upbeat, that is kick home the final 400m, so it is a good way to simulate a sprint finish at the end of a long race.
It is best to attempt threshold runs on fairly flat courses, to keep your heartbeat steady. Great places to try threshold runs in some of Australia's major cities include Centennial Park in Sydney, on the beach at low tide on the hard sand at the Gold Coast and around Lake Burley Griffen in Canberra. Bike paths along the Yarra River in Melbourne, Brisbane River in Brisbane and Swan River in Perth are all ideal.
I have recently been in Canada, mainly in the East coast province of Nova Scotia. A lot of the old railway lines have been torn up and local councils have put them to good use as Rail Trails. They are very flat trails with a dirt base and finely crushed gravel surface and usally travel between country towns in rural areas. Tempo running paradise! It is intended to be a fairly hard session, so it should be done after an easy day.