Please give some advice

without writing a training bible and not knowing anything abuot you, etc, here is some basic info.

without the proper base, it is going to be the easiest to get hurt, and thus most important to be careful with running, cycling and swimming are a bit more forgiving due to no impact.

running you want to build slowly, running more, not harder will allow you to run faster, but in order to run more, you need to slowly condition your body to be able to do this. the only way to do that is to slowly add more runs/time to your training. the rule of thumb is not more than 10% increase in workload running per week. again swimming and cycling are a more leaneant here.

the reason people say not to go hard is simply so you can train more. in other words, do enough work to get the job done but not enough to effect the next workout. someone doing 20 min a day (random number), day in and day out is going to be a lot better off than someone that does two hard long days, is beat up for 3 days, then tries it again.

going fast in triathlon (and other endurance sports) is mainly about upping the workload you can handle, recovering, and upping it a bit more. after you get to a decent, consistant volume, then its time to start looking at adding in some harder workouts. swimming/biking you can start that MUCH sooner, but at the starting point your largest gains will come from more training.