With the race being put into the game this could not be a much better way to change Gold into XP. The race will always cost you 50,000 gold and the reward is random, but hitting even 1.8m in a reward out of 20 tries would average the results of breeding metal dragons listed below. However, I am going to leave the information below as well because there is nothing saying you can not breed metal dragons while running races.
Based on the data I could find, raising dragons is the best gold to xp exchange granting a 4 to 1 ratio for Earth dragons and a 2 to 1 ratio for Fire and Metal dragons.
Behold, a chart! (Created with v1.7.2 data)
The above ratios assume you are not keeping the dragon and are going to sell the dragon as soon as it hatches. It also only contains dragons that can, at this point in time, be purchased directly for gold and raised.
There may be some long game plan you could do with raising farms to raise dragon levels but there is not currently enough information available for me to crunch those numbers to see if it is more efficient to sell a higher level dragon vs the food costs to get it there.
One last note selling and buying dragons every 5 minutes is not the least tedious thing to be doing, but it does cycle through your hatchery pretty well depending on what types of dragons you are attempting to breed.