If this is the case then our forces will play a defensive role and will force them to cross border to counter them inside our territory. Chinese can move their troops inside their land faster but this doesn't apply inside Indian borders because they'll have to face heavy shield of SAM network and our troops. So better strategy is to force them to enter inside our territory and then counter them in our land.
We should change our defense strategy from defensive to aggressive mode. If china can move 5 lac troops within a month then we should attain a strong air lift capability along with an effective air strike capability with pin point accuracy. We should speed our troops mobilization but also should get superiority in military intelligence so that we can know about such action to get buy more time for preparation. In short air superiority and air dominance with heavy air strike capability can counter Chinese half a million troops formation. Now a days war can be won only if we will have strong air dominance capability.