Hammers you asked the right question and the responses have been very good. But I would be a little bit wary with the fact that your test period is only for the past 39 days unless you have data from before this that franks your system.

Over 39 days you could probably find periods where backing every favourite returns the attractive figures you have quoted so I would recommend that you start off with a small bank.

Your win strike rate is also very high for the volume of bets stated unless you are using the market at very close to jump to determine your selections, this appears not to be case given your longer priced winners. A win strike rate of 31.3% with that volume is equivalent to the strike rate of favourites and in my experience there is no system or person that can select more winners over all races than what the market eventually determines is the favourite. So I would also be a little bit wary of your current high strike rate.