Okay, here's the plan.
Aug 11, evening flight from Mumbai to Delhi
Aug 11, late night train from Delhi to Chandigarh
Aug 12, early morning, reach Chandigarh and leave for Manali by road
Aug 12, afternoon, reach Manali; recce and finalize means for travel
Aug 13, early morning, leave for Leh by road
Aug 14, afternoon, reach Leh
Aug 15, celebrate Independence Day at Kargil (if possible)
Aug 16, 17, Trekking
Aug 18, Leave for Manali
Aug 19, Jump off Rohtang Pass, arrive Manali by evening
Aug 19, night, leave for Chandigarh
Aug 20, early morning train for Delhi
Aug 20, morning, reach Delhi, roam around, night flight for Mumbai
Sounds like fun? :)