I was an overweight person?

I was initially 86kg and my BMI was 30.8(obese).After going on a very strict diet of low carbs and aerobic exercises . i managed to bring down my BMI from 30.8 to 24.4, in about 5 months time.However , even though i lost all that weight , i still dont look muscular .So i carried on dieting strictly by not touching any fast food ,and snacks(chocs, ice cream ,chips and any non home-cooked food) and only consuming fruits , vegetables and only home cooked food.The main issue i have is that i feel very tired when i do my aerobic exercises when i am on a diet (Is it because im cutting down too much calories?).I dont get any muscles, i feel really frustrated.I feel so tired after doing the exercise .Must i eat more and hit the gym to get a muscled , buff body? and i am currently 170cm and weigh 68kg, how long will it take for me to buff up if i go to the gym ? Will i put on weight again once i start eating more (i am worried as was fat previously and DO NOT want to be fat again)?Can someone , perhaps a fitness instructor or a person that was formerly fat ,or anyone, please advice me?