How will I know when im ready for a marathon?

Yesterday i finished my first half marathon at 1:40:53 and i felt somewhat ok. My breathing was fine and i was even able to pick it up on the last mile, my legs and it felt like the rest of my body was getting very tired though. Am I physically ready for a marathon and if not what should i be doing to get there?|||Just keep working on endurance, the tiredness is probably the lactic acid build up.