It is very difficult to marry a friend and the main reason being the normal mutual respect friends have. That kind respect usually comes with great fear of disappointing the friend.
Whereas marriage/dating comes with a 'hit and miss' kind of characterization. You fall in love but you don't really know if they feel the same about you- so you make up your mind and try, if it goes through, well and good, if it doesn't, you move on.
It is never wise to keep a lady as a friend for so long, when the friendship respects settles, it would too much of a heartbreak to disappoint her and always the thought of loosing a friend would be great and weigh you down when it comes to making the decision of moving from friendship to a relationship.
Though friends have married- it is so much possible, the reason they succeed is that it happened they felt the same for each other for so long.
You can try but the good advice is always- make known your intentions from the word go.