Hi - I was diagnosed type 1 in 1982 and ran the London Marathon for the first time in 1983. Since then I have run numerous 10km and half marathons, 5 more Londons, 6 Beachy Head Marathons and even tried some triathlon including sprint and Olympic open water. In 2010 I completed the London Marathon in April (for JDRF and Diabetes UK) and the London Triathlon in August.
From single inections and glucose tablets to multiple injections and a whole range of "runner friendly" foods, things have certainly improved in those 30 years. I have never run to a formula of so many miles = so much food, but feel happiest to blood test before starting and react to how I feel on the way round. You learn what works by experience I'm afraid. For shorter runs of 10km or less I just carry jelly babies (in a bank coin bag with a foldover). I used to use glucose tablets but find them so dry to taste. For middle distance distances I often carry a running specific drink bottle (sort of split in two to make it easier to grip) with one of the powder mixed sports drinks. Current favourite is PSP22 blackcurrant flavour. Its a little bit sweet to taste but packs in the carbohydrate so a 500ml bottle contains about 40g CHO. Lots of other flavours and brands to try so you ought to find one you like. I hate Lucozade Sport but thats just me. You can also make you ow from fruit juice watered down 50% or Glucose Powder and Fructose powder mixed to taste. Drink is easier to take on during the run, and you need to drink for longer events anyway.
For longer events I find a mix of foods is better so thats where the jelly babies come in as well as the drink and then whatever else I can carry which will include gel sachets for use and also for emergency. I have also discovered Maxim screw top gels which are resealable and so allow you to keep topping up. The advantage of these gels is that they are concentrated and relatively small to fit in a zip pocket. By contrast the Beachy Head Marathon (which is a long distance walkers event) offers squash, water, biscuits, cut up mars bars, cut up banana, fruit cake, buns and I find this works well for me too - although you do stop at the feed stations so as not to litter the South Downs.
So you have to try it before the event to see if you like the taste and can stomach it (especially at a race like London that provides its own sponsored drinks on course), and you have to experiment with what is easy to carry and what works for you. I am still surprised with how much CHO in total I need to balance a run - can be be 50 - 60g CHO for an endurance event like the Grizzly which is hilly off road 20 miles taking me over 4 hours.
Good Luck - its the only way to spend a Sunday morning, and it keeps my blood pressure, resting heart rate and Diabetes Hospital Consultant happy.
Paul.