This is a possible route for the Community Marathon, which may or may not be run in Canberra on Sunday 11 April
The start and finish is in Lennox Gardens, which is near the Southern end of Commonwealth Avenue Bridge. It's a nice place, with lots of free parking, barbecue facilites and toilets. It's also where I got married.
The crux of the route is a 7k loop, which is done multiple times (3 times for the half marathon, 6 times for the marathon, 7 times for the 50k). The loop is actually a 3.5k out and back. It starts in Lennox Gardens and has 2 parts: it extends 1k down to the Southwest (toward the yacht club) and 2.5k to the Northeast (under Kings Avenue Bridge).
Note that the mapping was started at the Southwest end, near the yacht club. Lennox Gardens, and the start of the race, are at the 1k mark
The mapping is a bit rough, with the route apparently going in the lake and on the roads. Here it is in satellite view:
Having the loop an integral number of kilometres means that consistent kilometre markers can be put out so the runners can gauge hey they are going. The downside is that a bit of a twiddle is needed to accommodate the extra bit for the marathon/half marathon. One way to do this is to add an extra mini loop (just 98m) at the end. This will be run either once (for the half marathoners) or twice (for the full marathoners).
For the finish there should be a separate finishing straight, so that it is clear that someone is about to finish. This straight can cut across the grass and finish in the middle of the gardens:
An alternative to the mini loop (which I agree is somewhat inelegant) is to marginally extend each lap by 32.5 metres. Then the marathon is exactly 6 laps, so no extra twiddle is needed. However, then we can't have sensible kilometre markings (unless you are happy to have the kilometere marks 1005 metres apart).
Other distances are easily handled: for the 50k, do 7 laps. Then do another mini out and back for the final 1k (this will take you North, most of the way toward Commonwealth Avenue Bridge, then come back and enter the finishing straight).
5k and 10k can both be easily accommodated given that the loop is an integral number of kilometres.
Note that the course is pancake flat. An alternative is to change the 5k Northeast bit to cross Commonwealth Avenue Bridge and into Commonwealth Park. This would probably be more interesting, but there is a steep rise at both ends of the bridge - by lap 5 or 6 we will really be feeling it.