Conversions are difficult because the two exercises make very different demands on the body. Running is a weight-bearing exercise, cycling is not - most of the time.
In terms of energy expenditure, however, it is possible to make a comparison. Cycling at 12 mph consumes a similar amount of energy per mile as does walking at 4 mph. At faster speeds, the gap narrows because the air resistance becomes more significant for the cyclist - it takes much more than 20% more power to maintain 25 mph than 20 mph on a bike for this reason.
However, if you want a rough equivalence and your marathon-running GF does her marathon in four hours, she'll consume as much energy as she would in riding about 70 miles at about 18 mph. But she'll feel much more beaten up by the marathon because of the wear and tear of impact. On the bike, the machine carries your weight and you just have to propel it along.